The thing is it can be dialed out, it has been with after market exhausts. I dont think it really needs anything as complicated as an Exup or expantion chamber.
What got me looking at modding the standard system is what people are doing with the Stock RSV system, that is
here! It seems to work, everyone said they felt a change and there is one Dyno run which shows it does on last page.
Anyways, I think its basically the expansion type thing as the pipes come together that is causing the 5K problem. Think its worth getting a flea bay system and playing with it?
One other thing. What if you ran each exhaust independant, what effect would it have?