I am not sure how long of a muffler will fit under the Commander. But I am assuming there is some space. My personal experience:....
The Commander has the Gas Tank on one side and a very long muffler on the opposite side. So the dual exhaust is only dual till it gets to the single very long low restriction muffler with a single tail pipe.
So there is no room on a Commander for the traditional "dual" exhaust, where there is exhaust piping going down each side of the vehicle, on the Commander it all has to run down one side of the vehicle.
Before you get into moving the gas tank, it won't fit anywhere else, maybe you could spend tons of money (more than the exhaust system itself) to find a new spot for the spare tire, and put a custom gas tank in the spot the spare tire was. Then you'd have to live with the constant check engine light (CEL) being on, because I doubt you'll ever get a custom tank to replicate the evap emissions test results of the OEM tank and thus it will constantly have a CEL for failing the evap emissions test, even if you have the custom tank properly working with an evap emissions system.
So, a lower restriction muffler would improve performance, but NOT as much as a true dual exhaust with two low restriction mufflers.
And someone had a link on one of the threads with a picture of a true dual exhaust setup on a Commander that runs down one side of the Commander and fits two mufflers within the generous space for the single muffler.
It used two shorter low restriction mufflers with offset in/out pipes, so you stagger the tow mufflers, one after the other, using the offset in/out pipes to the muffler (i.e. one muffler flipped over) so the two pipes run parrallel, while matching up to the in/out pipes of the mufflers without having to make massive bends in the pipes. Then custom dual tail pipes over the axle to exit in the rear all on the same side.