On many race tracks the allowed DB values are mostly below the values that a normal racing system gives. The risk of not being allowed to drive after the long journey is there. Another possibility to bring down the DB values is to have an additional DB killer. This is not mounted in the muffler but in the connecting pipe. It can make up to 3 DB in one measurement.
Most racing exhaust systems have an outer diameter of the connecting pipe or inner diameter of the muffler of 65mm or 60mm.