Allfat -

I'm currently running an O.S. .91 Surpass 2 (4-stroke) w/a 12x8 blade in mine and have unlimited vertical at 3/4 throttle. So you'll be fine in the 'power' department with an O.S. .81 in my opinion. And as an FYI:I mounted all my servos inside the fuselage and shifted my Rx battery around inside to balance it out. Ididn't need to add any weight so my guess is you'll be alright.

Oh, most times I do all my aerobatics at 55% - 70% throttle. Iknife edge around 70% almost perfectly w/no mixing (However, I don't believe in mixing or dual rates - that's another story).

Cheers!