The Firebird and Deluxe mini-HBs are the same size, so no problems there.
I don't know if BKP actually make Firebird pickups, but Tim could definitely rewind your Gibson(?) ones for you.
As for how powerful it can be, I know Duncan's SM-2 Custom mini-HB is about 16K, but I have one and it doesn't sound particularly powerful (or good for that matter!). There must be limits due to the small size of the coils.
I'd ask Tim and see what he thinks.