custom wrote: if you remove the cats that frees up nearly 40bhp on its own
er no it dont!!! 5 -10bhp if your lucky!!
not saying you cant get power out of these engines, i love em, had a fair few cars with em in now (including a Dodge Viper replica, off the road with a destroyed gearbox.

) and tinkered with em too, i have two granny cossies at the mo, ones decatted and converted to manual, back to back on a rolling road it made 196bhp with the cats on and 202bhp with em removed and no other mods.
there are a lot of myths and wild claims for em though, but i have NEVER seen a N/A one make more than 250bhp running the standard BOA plenum despite the claims you see in mags, it just wont flow enough air.
there are a couple of aftermarket plenums available (i think top boss do one) and there are guys working on adapter plates to fit the later BOB plenum to the BOA base plate.
i seen the twin turbos running 600bhp, but as i said, those running grass track and saloon racers with em in all stick around the 400bhp mark as there are no reasonably priced gearboxes that will take that sort of power without exploding.