I've been noodling with a hermetic sahir for a potential upcoming campaign and wanted to bounce some ideas around and get other people's opinions, as well as figure out what hermetic sahir are actually capable of.
In brief, Sihr requires two separate rolls: One for the summoning (Presence and Sihr based), and one for the Bargaining (Bargain and Communication, plus Summoning Strength). Summoning Strength is the Summoning Total minus the Might of the jinn involved.
My first thought on this is that prioritizing the Summoning Roll is way, way more important than the Bargaining roll. Virtues that affect the summoning roll basically automatically boost the Bargaining roll through Summoning Strength. Also, boosting the Summoning Roll boosts Penetration directly. Puissant Sihr might only give +2 but it's valuable, as is Affinity with Sihr. While I'm on virtues modifying Sihr, Cautious with Sihr seems rather good to have, given Hermetic Sahirs are Twilight Prone. Get a familiar with a gold cord score and you can probably ignore most potential Sihr botches entirely unless you're throwing extra vis at the problem.
Also, since Sihr isn't a social roll (while Bargain is), the penalties of the Gift won't apply to Sihr but might apply to the Bargaining (The Cradle and the Crescent certainly states this for regular Sahirs). This still makes The Gentle Gift, Inoffensive to Spirits/Magical Beings and/or Alluring to (Beings) valuable virtues for the Bargain roll. Inoffensive to Spirits is probably the cheapest option to avoid the -3 Penalty. Oddly, since Sihr is a Presence roll, Alluring to (Beings) should still affect it, even if it's not a social interaction, and it will affect the Bargaining roll as well, meaning Alluring to (Beings) is potentially very valuable indeed.
Characteristic wise, Presence and Communication are very close in terms of priority, as both will contribute to the Bargaining roll (Presence through Summoning Strength, Communication directly). Given they're bought on a pyramid cost, having them equal each other is most efficient, but if I was to weight one characteristic, Presence would be higher priority (Penetration), even if Communication is probably more valuable to magi in general (writing/teaching).
In terms of regular Hermetic Magic, well, Minor Magical Affinity with Jinn is what you're stuck with, so you've probably got two routes to consider; either Elemental Magic or Vim specialist. In either case, probably Rego specialist, so you can ward/control Jinn through Hermetic magics. You could go generalist as well, but your focus is pretty restrictive. Obviously you'll want Second Sight, but doesn't everyone want that virtue?
Now, what can you do with Sihr? In general, the math pretty much says you can Summon and Bargain stuff with Might equal to your Sihr score without too much trouble. If you optimize for Sihr strongly, you can probably go almost vis-free (minimum 1 pawn) as long as you don't get greedy on the bargaining. Something like getting the Jinn to instruct you (really it's a significatos effect and should be governed by the Free Study virtue) for one season should be pretty reliable at Sihr=Might. If your Sihr is 5 or more points higher than the stuff you're summoning, it's cheap minions, even if you're limited by your Leadership.
If you took Free Study and Affinity, thats 9xp@Might5, 14xp@Might10, 18xp@Might15 and 23xp@Might20 per season, in either Sihr, (Realm) Lore or (Form) Art. With just Affinity it's 5/9/14/18, so probably not worth doing until you can reliably summon Might 15 stuff, as 9xp/season is kind of low, relatively speaking. The good news is so long as you can keep finding new Jinn, you can keep learning, and Jinn come in all varieties of (Form), so even without Affinity, you can still get a Might20 Jinn for 12xp, or 15xp with Free Study. Hermetic Sahirs definitely benefit when the books start to dry up, especially where (Realm) Lore is concerned.
For everything else with Sihr, it really depends on the Jinn you're dealing with. Most can do versatile stuff with their associated Art, and have Abilities and can fight for you and they don't even die if their corporeal form is 'killed'. You can also use Sihr just to summon stuff almost vis free for other purposes, like Spell Binding, or, if you can get them, Hermetic Empowerment or Spirit Familiar mysteries. Though you could summon stuff the normal hermetic route, Sihr is vis efficient compared to Hermetic magic.
If anyone has any thoughts/experience to contribute, please chime in!