Hmmm, found out the car has a boost controller kit fitted. As far as I can tell the standard boost is 9psi and the controller can provide up to 1bar (14.5psi) so that should bring the output up to the level it was as new (or maybe a little bit more). Don't really know exact gains from that increase...

So, this raises the question of insurance. I don't think I'd be comfortable with not telling the insurance company (and, of course, it's illegal ) so it looks like I'm gonna have to say something. Anyone got experience with this sort of thing (i.e. how much extra, will they even give me cover)?