RE: Minimum power for a 9# plane
I ran a 13x5 on my 55AX. Sounds like you're under propped. You can also try fuel with more nitro content.
Personally 12000 revs sounds food to me, bigger engines spin slower. 12000-13000 is good for a 46AX. 10000 on a 1.20AX.
APC will give you the most thrust. EMP is a decent cheap prop and it is black. I would also run a Coat woodie. NOT a Turnigy, they are unfinished pieces of junk.