When I want to create a custom event with UNIT_POWER_FREQUENT, I figure that it will bring some performance issue.

The current WA I'm making is for shadow priest, which has 2 types of power: insanity and mana. I want to do updates whenever my insanity is changed, but I also wish the mana change event is ignored. My current solution is manually check second parameter (power type) every time the event is trigger, but I hope that you can add some parsing for this specific event (maybe related to RegisterUnitEvent?)

What you're doing is correct.

That's how the in game api works. There's little to be gained to further filter that on the WeakAuras side instead of your code.