only a cat back?

hi folks got the STI just over a month ago with i believe only a H & S cat back, looking under the car from the middle silencer, back its shinnier,than the front sections. however was at a rolling road the other day with scottishscoobies.net and got 318bhp and 320bhp, is this right for JUST a cat back, wouldnt have thought it would have gained that much?

any ideas if thats right?

The Prodrive kit - remove 2nd cat pipe and rear box, changed ECU/map, gives 30 bhp.

So changing exhaust cat back may well get 2/3 of this ... air filter still std?


Others on here have changed back box or cat back, some may have figuers.