On my G-26 I ran a couple tanks of fuel with an APC 17/6 airplane prop on the bench to run in the engine before I put it in the G30 Radikal.
I did some tach readings with the stock exhaust can, the Torpedo muffler and with and with out the stock air filter.
My readings showed around 8880 rpm with the stock muffler and stock air filter and around 9450 with the Torpedo muffler and stock filter.
It didn't seem to make much difference if I had the baffles or insert in the muffler. It also didn't seem to effect it if the stock air filter was on or off.
It may have spun up a little quicker with the insert in the muffler which would make sense cause of a little more back pressure but the overall top end didn't seem to change with or without the insert/baffle.
I have since installed a velocity stack air filter from Dave's Motors on the heli and it seems to pull better in the air with it but I haven't done the airplane prop test on the bench with it yet.