Here's the proof in the pudding that nigh on 300bhp is easier than you think...
This was a Lemans model with 8:1 compression, large T3 turbo, remapped ecu (only lemans model can be reprogrammed) and a set of the 270 cams that I have on my site. This car was running 1 bar of boost, as this is still the max the factory ecu can take.
286bhp and 310lb ft of torque. With the same cams on a 8:6.1 compression GTA, it made 330lb of torque at 3500rpm. Downside is it makes it a very cammy car..
Now even more gains could be made if you used one of my straight through exhausts and intake, as the car above was running a Devil system, which is hardly any better than standard,.. and proper aftermarket management to run more boost with better injectors.
It seems no matter what you do though, 300bhp is the limit of the manifolds whether it be a GTA, 610 or Venturi. Europa cup cars made no more than 290bhp, the 3.0 PRV as used on the Venturi and A610, only makes 281bhp with a remap and 1 bar of boost, but all engines make alot more torque..