Can't say how accurate the rollers were but other people were there at the same day ( interpro ) and they had been on other RR's with very similar results to what they got on the day.
My old CVH was a completely standard 1600 cvh with just the compression ratio lowered to 7.2 -1 by skimming the pistons and using a brand new un skimmed head, the head was un flowed, and standard size valves were used, a standard inlet and t3 manifold were used, also un flowed and the turbo was a 4x4 cossie one with a .63 exhaust housing ( not cossie one ) i had a GRS intercooler and ran 1.5 bar, it made 255bhp and 253Lb of torque and wasn't really laggy at all!
As lemon says, Tom Scarletts has also made 300 so it's not impossible