Hallo!
I have run several of these Irvine minipipes ( Magic mufflers) when we flew Sport .40 pylon here in sweden a couple of years ago. We used APC 9x7, 9x6,5 or 9x6 props running on straight fuel my engine rewed appr. 17200 on the 9x6,5 and 9x7 props and 18500 rpm on the 9x6 APC.
The pipe were made for the Irvine Sport .40 engine!
These pipes are made to work att 15.000-20.000rpm so do not try a 10x6 on them ( on a .40 engine). it will not get "on the pipe" .
It might work though if you use a ,53 engine. But remember that you have to let the engine rew above 14000 to get the pipe work!

Regards!
Jan K
Sweden
27 years in R/C