That what he has said to you mate?? The car is running a 2.5 with a VF22, and at 8psi, which is only about 0.6bar of boost..... I cant see how he's making those figures?!
A car like that will cause you problems, if nothing else, it will break things from stress etc.. I have no doubt that its probably a pretty frantic drive and will feel WAY quicker than your RB does at the moment, but with the water pump problem, the wild power claims and the way that he's not prepared to fix what is a relatively small problem considering the size of the engine build its had.. I'd steer clear cause you could be buying someone elses problems here mate.
If it was local-ish, it'd be worth a nosey, but Leicester is a fair old trek for something that is more likely to be a non starter... pardon the pun.
Cheers,
Grant