Other than getting a ticket, perhaps nothing. Mufflers aren't really a source of back pressure. The cats are. Everybody keeps sayin the JK motor needs back pressure. No engine "needs" back pressure. It's not about back pressure. It's about exhaust scavenging.

This is the effect that the right size diameter pipe has on an engine. Make the pipe too large, and scavenging is all but lost. Too small and the exhaust becomes restrictive. There has to be some scavenging in order to clear the cylinders of some spent exhaust when both valves are open. Back pressure always has a negative effect on an engine.

An engine is nothing more than a self powered air pump. If it has to fight to move air, then it is less efficient. Everything from the cats back is there for two reasons. One of course is to reduce sound. The other is to help reduce the exhaust pulses and smooth out exhaust flow, to help aid in scavenging and to give some lower end torque.

Short exhaust and tail pipes may increase upper end horse power, but they also make low end torque fall off. The JK needs all the lower end torque it can get. You can prove my point at any comptent exhaust shop. Let them measure back pressure before the cats with and without the rear exhaust on. Either there'll be no difference, or it will be extremely small.

So the best case would be to try to find the freest flowing muffler, but retain the stock diameter pipe so that the scavenging effect will not be lost. there's a forum member who backed all of this up with dyno runs. He concluded that a good free flowing muffler with single exhaust was the best in terms of power and torque. The work's already been done in this argument. Nothing else to agrue over.

EDIT: here is the thread with the dyno information. We need to thank 1BADJK for all this great info.

https://www.jk-forum.com/forums/jk-write-ups-39/results-848/