Umm....

That's not all that far off what it should be. What percentage nitro fuel are you using? I've got a couple of these little beasties and can get about 9500 max static RPM on an APC 16x6, so if you accept that going down an inch in diameter and up one step on pitch is roughly equivalent, you're pretty close but the nitro percentage will have some bearing on what it'll turn. Have you checked the valve clearances? That'll make a difference as well if those are off, usually too loose.