These engines aren't known as power-houses but, an 11x6 is on the small side for a .61.
My TT.46 turns one at 13,5. I wouldn't expect too many more revs in the air than on the ground at WO with that prop. It has to be screaming near 16K!
The Fox plug is the other problem. The manual warns against them and none have worked in any of my many TT and low nitro fuel will just make flame-outs worse.
You may have better luck with a 12x6 and an OS A3, #8 or K&B 1L.

Kirk