The TT muffler that you get with the kit is relatively quiet, about half the noise of most airplanes IMO. If you decide to make it even quieter there are a couple things you can do: 1) make an insert for the exhaust so you get a bit more back pressure. make an aluminum tube that is a press fit to the inside of the outlet and restricts the flow just a bit. This will hurt performance, but there is plenty of power to spare. I was messing around with one of the MP2 tuned pipes the other day, it is quite loud but really quiets down if you do that. Haven't flown it with the mod so i don't have any hard numbers.

the second thing you can do is make a second chamber for the exhaust that has baffles. similar to a gun silencer maybe? a little more work, but it might quiet it down with less effect on performance.