For the STi, very. Bearing in mind they're 260bhp stock, we had the PPP on ours which is nothing more than a remap, deleting one of the cats, different backbox and a bigger fuel pump. This gave our car 319bhp, and that's on stock injectors and intercooler. Go for an FMIC and bigger injectors and you'll get a fair chunk more.
Gearbox on the STi is also much stronger (read: bulletproof) than the WRX version, so worth considering too.