You know we are facing similiar problems here in the states at various race track events. One local "Hill Climb" I'd like to do this summer requires 95 DB, thats pretty darn quiet! Other tracks are around 103 DB, it varies.
SO, I contacted "Toms Motorsports" in California, they race, they been having this problem. They've tried all kinds of "things", it either takes away a LOT of horse power or builds up to much heat in the engine.
They are now using Spiral Turbo Baffles with great success! Limited horse power loss and ADJUSTABLE DB!
http://www.spiralturbobaffles.com/index.html They also put a quick release out let tip on the end of the side pipes they can "turn", down,out, left, right, depending on where the sound reading is being taken at any given track.
I HAVE to get a handle on this sound issue, as everybody will eventually. I'm looking real hard at the Spiral Turbo stuff.