Went to Silverstone last Saturday with a Radical with full race silencer and didn't have any problems, but we did limit the revs to 9,500 which is 1000 rpm below max. Another Radical did have problems, however, but was given a chance to rectify matters. The noise regulations have definitely tightened up - due to a local protester who has got the local council involved apparently. Why is it that councils give a lot of attention to tiny minorities and totally ignore the needs of the great majority?
Anyway, the only measurement is drive-by, 102db I think, although a static test is voluntary if you want to get some idea of the noise output of your car before taking to the track. I would have thought that a GT40 that can achieve 105db static shouldn't have any problems at Silverstone. F1 cars are exempt from the noise regulations on their test days!
Chris