The golden rule here being that you should almost never believe anything that an RR has to say.
Maybe you can trust it if it's a brand new, top of the line Dyno Dynamics but if the geezas got some old sun roadamatic or bosch 202 in his floor that hasn't been calibrated in 10 yeas then I'd be walking away
As the guys have said, 240hp on a standard internal engine is about the limit. Even that takes some accurate tuning, a good cam and a T28 to achieve. Mart always used to do very well at such relitively small amounts of boost, but most folk find it hard to make 240hp no matter how much boost they run.
Stuart clark has to be No.1 carburetted C1J BHP guy in the UK. He will have had an honest 300+ hp to make the trap speeds he was doing. As he prooved to himself though its nigh on impossible to go any better than that even when he added nitrous and found he went no quicker.
Finally now that we have an EFI powered C1J owner willing to push (or should I say let me push !!) his engine to the limits (Glenn HI5) we are able to see what additional power the C1J can make with the tiny carburettor removed. Glenn's engine is pretty hardcore in terms of build but essentially still runs a standard-ish cylinder head and a common as muck Piper 285 cam at standard timing. It makes 270hp or so @ 32psi boost, day in day out (you can literally pin your foot to the floor in 5th gear, get right up to the rev limiter and not have to worry about anything letting go!) and about 400hp with nitrous switched on. With a carb that would have been very hard to do.
What interested me the most with EFI on the C1J is that up to about 25psi of boost (in Glenn's case) the power output was comparable to that of a well setup carburettor engine. But where as most carb fed C1J's refuse to make any more power past that sort of boost the EFI engine would just keep going and going, responding to more and more boost. We've been able to go right up to the point where reversion has become the biggest problem and so now we're trying a new (actually my old C1J) camshaft to see if it'll help with that.
There you go, probably the longest post I've done for months