The issue is I still don't have a product to test, I can't find one suitable. I know mine isn't legal. I will take you up on that offer when I sort it though if you don't mind...
The best idea I could come up with is to run a secondary 2 inch system/muffler after the diff, with a spring loaded flap in the large pipe, to block it on cruise and idle. Or perhaps an electronic butterfly valve like below? I also have a helmholtz chamber here I designed that I want to try.
Anyway, I guess this conversation should be moved to its own thread, sorry mods.