I think you're back to just using the engine size and type plane, Delta w/up to .10, delta .11-.25, delta .26-.40, delta .41-.60etc and Airplane with stabs(elevators) etc... It doesn't matter what engine size was recommended, only what engine size is used. Then the .40 powered whip doesn't fly against the .91 powered whip. And the X class would be like the big dog trophy class.... You won't be able to please everyone so don't try. Make up the rules and if the poeple want to fly they'll show up... If I was doing it I wouldn't even seperate the types of airframes, just do motor size classes.

I've seen many, many, many slower planes win simply because the faster guy wanted more power, leaned her out and ended up slower.