Not after the additional few bhp per se, more after a smoother delivery and removal of the limiters. Am also going to fit better exhaust (prolly Eisenmann) and think that the two will make the car sweeter.
CA have done all the work on my E39 M5 (PBMW mag featrue car Dec '08). very high quality work. they have also done a custom map on my car, cant recommend them enough. ask for Roy or Phil.