Correction: a piped engine needs to rev in the rpm band the pipe is set for...

But yes, a 10x8 MA is quite a big prop.
You say you got a "pretuned" system... that leads me to think it is tuned for more rpm than you are getting on the 10x8. What are you getting on the 10x8 by the way?
A 9x7 - 9x8 - 10x6 prop should get you in the ballpark.

Watch out with the needle settings... don't go for the absolute peak like you can on a muffler. If the pipe is set correctly it will unload a lot more in the air than it does on a muffler... a very lean needle setting at this point will kill your engine.

If your engine is smoking after you shut it down it is WAY too hot... you may have already smoked the engine ... they really don't like getting that hot!