id say you'd be lookin at an increase of 20-30bhp. depends what chip you mean? dont go to superchips as ive heard on here that there shit, also alot of other ppl will tell you the same. go for a re-map of the standard ecu or a dastek unichip. get it de-catted too.
i have a 4-2-1 manifold, full de-cat system and piperx viper.