I'm in the peculiar position of knowing where a 68 Might Earth Jinniyya is located. I'm also in the same peculiar position of having access to some 22 Terram Vis in the stores/nearby I might reasonably steal or bargain for.
According to the Sahir rules, I do not, in fact, need to summon her first in order to strike a Bargain. The spare 8 Vis and a no botch on the Bargain guarentee I could bind her to do anything I wished for a year. Further, she can't attack me (not that she's inclined to) or leave while the bargaining is going on. This would permit me to collect on a one-time each benefit of 42 experience in Sihr (an accelerated ability), Faerie Lore (if I bothered), and Terram, in addition to doing everything I wanted. (Could get her cheaper if I just wanted her to let me learn Sihr for a season, which would be crazily beneficial in itself.)
Frankly, though, this seems too cheap, especially for troupe play. Like being the Storyguide and handing yourself a superweapon (though I did not produce the Jinniyya or even suggest her.) I think it may be reasonable to impose some sort of Vis restriction (not normally applicable since this is merely a gift to butter the spirit up) or just artificially prevent myself from taking advantage of it.
Might 68 is crazy-powerful. That XP is roughly equivalent to 6 or 7 excellent Tractatus, but consumes 22 pawns of vis, only works for you, and can't be shared. And, of course, the fact that she's not inclined to attack you now might not hold true come the end of the bargain.
Err.... does GIVING the vis to the jinni as the result of the final deal count as USING the vis? I wouldn't think so, but no idea about what the rules say. I always thought about it about giving the jinni some sweets for his entertainment, so you do not have to manipulate them at all.
You are effectively treating with archangel jinni here, mind you. Quite an entity. Does she rule the whole of North Africa or something like that? That power level would imply that to me....
About the power level, spending more than 2 rocks of vis is massive. At least IMS. Getting 3 XP per pawn (roughly what you are saying) sounds OK to me when it comes to power levels. THe advantage you get is the speed in evolution, but you are unlikely to get 20+ pawns of vis to repeat the feat in a while, so it does not seem game-breaking. Besides, sahirs are non-game breakers by definition as far as I can see from an overview of what they did. I am missing some of the game mechanics that can break them, I guess, but when I read the rules they seemed OK as an exmisc tradition that had HUGE limits on their magic.
Just to get my hands on 42 experience, it may be worth it to do it for a season. However, those are very interesting assessments, particularly the one about Fae politics. We know she has a serious and powerful rival.
And yes, we're located at one of the more powerful auras in existence around which she lives near. (The only reason we aren't swamped in magi is because of the six or so covenants established here, not one of them has survived due to Mysterious Circumstances. We're the metaphorical Canary in the Coal Mine down here.)
EDIT: Ah, in addition, she wouldn't be liable to attack me afterwards either unless I seriously pissed her off somewhere along the way. The bargain itself is considered 'fair.'