A lot of aftermarket hi-flo cats get "eaten up" and end up needing to be replaced, but it has nothing to do with the header.

I would recommend a carsound hi-flo cat; unless you don't mind running a test pipe 24/7. Gutting your stock cat (unless it's wrecked and rattling) isn't really the best way to go about things.

Overall, a hi-flo cat coupled with I/H/E works as part of a system and will net you some pretty good gains. (I've seen 10-20 whp on a stock B18c1)

Shingo