Interestingly I was down at my local MOT garage the other day for my missuses car.
Had a look at an M5 with the bonnet up, talking to the fitter, he said it pushed out 720 HP (!) had been tuned to death by them, and looked pretty impressive (not for me though)
It is also the local tuning garage, the fitter told me the owner worked on James Hunt's F1 motor back and when, and that's where their expertise comes from.