You have to take what you read here with a grain of salt. One man's rocketship is another mans slug.

I've flown my skybolt with an OS .75AX spinning an APC 14x6 at an even 10,000rpm and the plane flies very nicely indeed. The vertical is excellent. It's not unlimited but it's almost there. My own opinion was the model was perhaps a little too fast so i contemplated slowing it down with a 15x4 but never got around to trying it.

I'm sure the .91FX would be even better. It weighs the same as the .75 and has about 20% more power.

I'm now flying my skybolt with the OS .91 Surpass II 4-stroke and find it slightly less powerful than the .75AX with the APC 14x6 (400rpm less) but with a Graupner 14x7 it flies very well and with more pull than the APC 14x6 and 14x7 props (those graupner super nylons pull like a tractor).

I agree with what has been said about the CG. Mine is also a bit further back than the recommendation.