For a while now I have noticed that it is difficult to get our 2008 MCS R56 to rev much past ~4300 RPM in 3rd, 4th, 5th & 6th gears. Acceleration is way down and the car has lost its usual punch. Also (and this just may be me becoming paranoid) there is a turbo like whistle at lower constant revs. The dump valve can still be heard on changing gear and I think we are getting more crackle and pop from the exhaust (no complaints on that front though). It doesn't appear to be using any more oil than usual and the acceleration is fairly smooth, ie no stutters and idles well. Took the inlet manifold off today and there is some carbon build up but I've seen worse and I intend cleaning it when I have more time. I tend not to think the issue is being caused by carbon build up as I do not have any of the other symptoms. But am willing to be proved wrong.
Searching around the interweb there are a (very) few people reporting the same issue but either they have bailed out of the post without saying how it was ultimately cured or the fixes entail taking the car to the stealers. I would rather satisfy myself first that it is not something I can handle myself.
Any assistance gratefully received.