The Flowmaster muffler is complete garbage! You will not see much if any improvement installing a Chokemaster muffler. Beyond that, their tech support are complete idiots and do not have any remote clue of anything about engines.
When I first bought my motorhome, it had Flowmaster 50 series. I installed Thorley headers and saw very little performance gain. I then had the exhaust completely redone with Flowmaster 40 series installed. There was a significant improvement in performance, however the noise level was unbearable. I saw parents hiding their kids, when I pulled into the campgrounds. I had the 40 series pulled, and had 70 series installed. The noise level was much more tolerable, but I lost over 2 mpg and 2.6 seconds off my zero to sixty times ~10%) After much wasted experimentation based on Flowmaster's tech support, I gave up and installed Magnaflows. My power came back, and my mileage returned. Not only that, but the noise level inside the coach was quieter than with the 70 series Flowmasters. If you are going to install an exhaust system, you need to have straight through muffler, such Magnaflow or Dynomax. I would also suggest installing a H pipe or X pipe, which can be purchased from Summit Racing. X-pipe is better if you have the space, but it need to be within about 2 feet of the collector.
While Gibson will give you a decent improvement, I personally am partial to Tri-Y design, as it will have a much broader power band.
BTW: Since I was kicking those 70 series Chokemasters around my garage for so many years, I installed on on my 6 cylinder Jeep because it needed a new muffler. I assumed such an over sized muffler wouldn't do much damage, but as a matter of fact, there still felt some power loss in the top end. It was an old Jeep that didn't matter, but I was surprised that it had a negative impact on such a small motor.
IRV2
