If you're looking at modding a car - which it looks like you are, then best thing to do is take it to a decent RR when standard and every mod you make, get it retested at the same place. It's the only way to be sure of know what differences mods are mkaing to power/torque.
If you want just a good RR specialist I'd recommend SCA in Norwich (JFGI). The main guy there, Steve I think, has his own 450Bhp S13 200sx and has it on the rollers everyday. So he really knows his stuff about the finer elements...