Hector (ADTR) is developing the first custom exhaust for the Bonneville. I think he'* installing the first one this weekend. He'* the best source for information on this, but I know for a fact he'* thrown around some various configurations for consideration. I believe his final design will be very much performance oriented as opposed to sound oriented. No ricer fart-can mufflers here.
You probably don't want to increase your pipe diameter too much. There'* a good chance you might loose low-end power if you sacrifice too much backpressure. Lean on Hector for more info on this. email@example.com
or pm 99BonnevilleSE here on the forum.