I drive a Honda, and replacing the stock cat (which is super restrictive) for a Hi-Flow makes a big difference. But yeah in an article I read the hi-flow made 5 WHP/TQ over a stock, but a test pipe/no-cat only made 1 more WHP/TQ; this was on a B16/Si.

On a turbo car I'd never run a cat though, it'd get clogged fast from all the carbon that turbo setups put out.