Hm. I guess I did not understand you. Shia gives +5% morale, and +10% morale in full mysticism. If you take a school, you get 10% shock damage, if you invite another school, you get -10% shock received. It is going to happen with Shia....
Full Mysticism is highly unlikely because every war declaration and conversion is going to move you towards Legalism
This, although I'm not convinced the conversions changing piety is actually working. But if you want to outweigh war decs via events, you get to declare war on a non-same religion target roughly once every 15 years, and even then you'll only move slowly towards Mysticism, resetting, of course, on ruler death. On the other hand, you can hit 100% Legalism in just 4 war decs, which if you're blobbing hard, is maybe a few years, maybe a few months.
So realistically Shia is 5% morale, shock damage, and legalism bonuses - which aren't bad. But there's no way you're going to max mysticism bonuses unless you're almost completely pacifist.
I wouldn't necessarily swap to Sikh from Shia - I can't remember the Guru bonuses and how they stack up vs. Legalism, but there's no particular reason to go away from Hindu, which gives you RCC/-AE right there (or disc if you need it), plus tolerance of true faith and heathens.