You're making a big mistake. Just get their muffler and do it at the same time.
I installed the pipes and left the stock muffler on and it improved the sound a little bit and seemed to help the engine rev faster.
Then I installed the Top Speed muffler too!
It sounds fantastic!
Listen here, look at the info for where to find the acceleration runs.
Now it revs faster, runs faster and even at start up, it makes me smile.
There is a drone in 6th at 2,400 to 2,800 so shift to 5th.
Very nice weight savings too, 30 pounds total.