After reading your last post, I think this is getting into conceptually uncharted territories for a storyline that we're not getting as much context to as we will need to make even a decent stab at what's going on metaphysically. (Say that three times fast.)
Anywhos, I'll circumvent that previous bit of information -- I honestly can't say what would happen because hellall if I understand it -- but as far as primal tempering this is how I've seen in-the game.
Mentally: As we are all aware, the Primals themselves appear to possess their own unique personalities and preferences. Not all Primals are made from the same cloth. You have Ifrit, who is the poster child of aggression and domination, but then you have Titan who honestly struck me as a nurturing father that is ROYALLY PISSED at the player characters for causing harm/killing his "children," then you also have Garuda who is both proud, confident, and at the same time insecure and vindictive (though that might be my own personal interpretation). You can also see that the beastman that worship begin to take on personality traits of the Primal that they have summoned. If I recall correctly, the Amal'ja actually had peaceable trade relations with Ul'dah before they summoned their Primal. Afterwards their aggressiveness and tendency to raid caravans for aetheryte earned them the gall from the Jewel of the City. I think the best example we see are the Sylphs. The Sylphs originally had no intention of summoning Ramuh, but fear and destruction to the Shroud made them do so. After they summoned him (though we are told he did not heed their summon until he felt that his divine justice was needed), his tempering drew them toward a collective unit and they brought vengeance and retribution to those who would threaten their forest. Unlike the Amal'ja, who are destructive almost for the sake of destructiveness, the Sylphs still maintain their quizzical and playful demeanor. Only now they are using it for the service and protection of their woods rather than for fun and silliness.
I think one thing that hasn't be touched on, though, is the fact that the tempering process effects the SOUL. This is a bit more severe than just touching a person in the mind -- though it appears they can do that too. This goes beyond just aligning a person's sense of what they want to do, but appears to go deeper to changing the fundamental reason a person was made to live. It is my belief that the individuals who appear shattered and broken due to this process are the ones that have had the biggest clash between what their heart and mind tells them they are and to what the change has made their soul TELL them to feel and believe. The ones that appear to handle this best are the ones that were more closely aligned to the primal in the first place. For these individuals it is not a change of their fundamental being but rather just a re-alignment of their desires and goals.
Magic: I don't think you are wrong in the assumption that the Primals provide a measure of their magic to their subjects. All of the Beastman Tribes that have summoned their primal has some kind of buff and magical attack that utilizes this aspect. The Amal'ja love to use Enblaze, Blaze Spikes and Fire-based magic. The Sylphs use Enthunder and Thunder spells like they were tossing candy. The Ixal use Enaero and wind-based evasion buffs. And the Kobolds usually have a Mini Titan's Wrath move which works a lot like Titan's plumes. Now, as for right now --I don't think-- the Sahagin have been successful in summoning Leviathan --so far--. I know they are trying to, but as far as I am aware the Lord of the Whorl has yet to heed them. That being said, there is also a camp of Tempered Hyur and Lalafells just North of Zan'hraak. And I don't believe they used Fire as much as their Beastman counterparts.
Communication: Of what i have seen, and if Hydaelaen is believed to be a Primal-like entity, the link between subject and primal is not two-way. I will agree wit the people that said that the communication between Primal and Tempered appears to have more in common with a one-way radio. "I will tell you what to do and you will obey." I suppose another video game analogy would be the Archdemon for the Darkspawn from Dragon Age.
Anywhos, I'll circumvent that previous bit of information -- I honestly can't say what would happen because hellall if I understand it -- but as far as primal tempering this is how I've seen in-the game.
Mentally: As we are all aware, the Primals themselves appear to possess their own unique personalities and preferences. Not all Primals are made from the same cloth. You have Ifrit, who is the poster child of aggression and domination, but then you have Titan who honestly struck me as a nurturing father that is ROYALLY PISSED at the player characters for causing harm/killing his "children," then you also have Garuda who is both proud, confident, and at the same time insecure and vindictive (though that might be my own personal interpretation). You can also see that the beastman that worship begin to take on personality traits of the Primal that they have summoned. If I recall correctly, the Amal'ja actually had peaceable trade relations with Ul'dah before they summoned their Primal. Afterwards their aggressiveness and tendency to raid caravans for aetheryte earned them the gall from the Jewel of the City. I think the best example we see are the Sylphs. The Sylphs originally had no intention of summoning Ramuh, but fear and destruction to the Shroud made them do so. After they summoned him (though we are told he did not heed their summon until he felt that his divine justice was needed), his tempering drew them toward a collective unit and they brought vengeance and retribution to those who would threaten their forest. Unlike the Amal'ja, who are destructive almost for the sake of destructiveness, the Sylphs still maintain their quizzical and playful demeanor. Only now they are using it for the service and protection of their woods rather than for fun and silliness.
I think one thing that hasn't be touched on, though, is the fact that the tempering process effects the SOUL. This is a bit more severe than just touching a person in the mind -- though it appears they can do that too. This goes beyond just aligning a person's sense of what they want to do, but appears to go deeper to changing the fundamental reason a person was made to live. It is my belief that the individuals who appear shattered and broken due to this process are the ones that have had the biggest clash between what their heart and mind tells them they are and to what the change has made their soul TELL them to feel and believe. The ones that appear to handle this best are the ones that were more closely aligned to the primal in the first place. For these individuals it is not a change of their fundamental being but rather just a re-alignment of their desires and goals.
Magic: I don't think you are wrong in the assumption that the Primals provide a measure of their magic to their subjects. All of the Beastman Tribes that have summoned their primal has some kind of buff and magical attack that utilizes this aspect. The Amal'ja love to use Enblaze, Blaze Spikes and Fire-based magic. The Sylphs use Enthunder and Thunder spells like they were tossing candy. The Ixal use Enaero and wind-based evasion buffs. And the Kobolds usually have a Mini Titan's Wrath move which works a lot like Titan's plumes. Now, as for right now --I don't think-- the Sahagin have been successful in summoning Leviathan --so far--. I know they are trying to, but as far as I am aware the Lord of the Whorl has yet to heed them. That being said, there is also a camp of Tempered Hyur and Lalafells just North of Zan'hraak. And I don't believe they used Fire as much as their Beastman counterparts.
Communication: Of what i have seen, and if Hydaelaen is believed to be a Primal-like entity, the link between subject and primal is not two-way. I will agree wit the people that said that the communication between Primal and Tempered appears to have more in common with a one-way radio. "I will tell you what to do and you will obey." I suppose another video game analogy would be the Archdemon for the Darkspawn from Dragon Age.
Thy life is a Riddle to bear Rapture and Sorrow
To Listen, to Wonder, to Entrust unto tomorrow
In One fleeting Moment from the land doth Life flow
In One fleeting Moment from the land doth Life flow
Yet in that Same Fleeting moment Thou must Live, Die, and Know