Derek,
I´m running 2 O.S. 50 SX at 9000 ft altitude with 15% nitro Power Master fuel, both runs perfect, one is giving 12,100 rpm with an APC 13X4W in a H9 Twist 3D.
The other one is giving 13,800 rpm with an APC 12.25 X 3.75 that is mounted in my GP Venus 40.
Both engines are fitted with modified Tower Hobbies mufflers.
Check this thread link:
http://www.rcuniverse.com/forum/Over...2648733/tm.htm
30% nitro will overheat your engine and will cause a lot of detonation, and as a consequence the piston can be melted or the piston pin rod can show early wear.
If you want more speed, try a lower diameter propeller on it.

Regards.