You need to 'reverse engineer' this queston.
First thing to investigate, of the tracks you intend to go to, is the venue which stipulates the LOWEST dB of the exhaust system. From there, you can then investigate the most free-flowing system, whilst at the same time meeting the dB limits of said tracks. So just turn up and hoon.
This wiil stop you having to faff around with running around with a bung in, etc (which could prove dangerous to your engine when running for extended periods at high rpm)...