Barry has some good points there,the extra 11/2 lbs would prolly ruin the plane for any real good 3-d moves ,but still be a good sport flier and acrobat.
Remeber this the mayhem is not designed for high speed it warns against this in the manual.
Although for short burst I open my 120 YS up and so far never had a prob.
The 91 FX will put out allot of power but will it 3_D the plane the way it was intended ????That I can't answer.