I have flown HS81's in some fast 15 size planes, including my Sig WOnder with the piped Rossi 15 FIRE and that thing WENT! Its not just the servo torque but the size of the control surface as well and the speed. Bigger surfaces or faster planes generally require stronger servos. I have even run the older HS 80's on a .25 size low wing pattern style plane with good results.
I wouldnt use an hs55 on anything larger than a true 1/2A myself (up to an 049) and even then I prefer the extra oomph the HS 81's have for anything larger than an 020 really.
Just my opinion though
AJC