13x8 is way to small a prop, this engine can turn more...

http://www.os-engines.co.jp/english/...e/proindex.htm

I would go for the 14x10 on this plane, remember you want airspeed to keep up the acrobatics. Also throw in a little nitro (10-15 %) and you will have no problem turning that prop. Anything from 8500 and up is enough (not more than 14000 though). And a tuned pipe WILL do wonders...

I turn a APC 16x10 on my OS 120AX (tuned pipe) (10% nitro) at 9800 rpm on a Groovy 90 - unlimited vertical