Glad to hear you have it running ok.

The Vmax engines have a muffler similar to the Tower muffler - semi-tuned exhaust.

The 11x6 was definately way too big. The engine has to turn up well over 14,000 rpm with that exhaust to run right. Over 15,000 would be ideal. You might even want to try a 9x7 and let the thing wind up a bit.

Worry not about the backpressure. The problem as you have seen was the prop.