IIRC there is no legal limit as to how load your car exhaust can be, the whole issue has been brought about by young lads with thier baked bean can exhausts thrashing thier 1.0 corsas about making a racket spoiling it for the rest of us, so due to complaints the police now keep an eye out and may pull certain cars with loud exhausts.
I am fairly certain this is correct as some TVR's M3's (not forgetting your exotica obviously) and even some bikes are pretty damn loud with totally standard systems on at idle nevermind at WOT!.
The reason why there are lower limits imposed on some racetracks are only there because they have to take into account local residents as the noise will be more constant with lots of cars flying around a track, if a track has a high or no limit chances are it will be a fair distance away from any houses etc.
