What are some nice humbucker sized p-90s you can get at guitar center except for the phat cat and the p-94?
I don't know about guitar centre, but GFS make a couple and Lindy Fralin (sp?) also. That I can name off the top of my head.
Fender Precision Bass
Fender Jazz Bass
1967 Fender Coronado Bass II
Warwick Star Bass
Squier Precision Bass TB
How much output does the Kent Armstrong Retrofit P90 has?
Is it any good for punk?
You might want to look online before buying from GC they do not have much of a choice when it come to pick ups. They will tell you they can order stuff but you might never see what you order. I ordered a set of pups thru them they normally do not carry and 6 months later still no pups. I canceled my order got me deposit back did an order on line and had my pups in 4 days.

There are a lot of better choices out there. I really like Rockfield but I do not think they do any P90 pups at all. GFS makes great pups I really think they are the best bang for the buck outside the Rockfields.