Since this is the aerodynamics forum and IE has said let's go for broke and DARN to the rules we can toss out the AMA restriction on turbines for whatever reason they put it into place. To a great extent a lot of this will be theoretical in any event.

There's no doubt that the power source is 95% of the equation on a speed model. The Extreme Speed Prop forum is full of models that fly very fast but do not have the best aerodynamics by any means. Brute power is obviously the cornerstone. But given two identical brute power sources what is needed to make one go a lot faster than the other? Now we come down to cleaning up the airframe so that the engine can spin a higher pitch prop and still unload enough to get up to the peak HP figure. And THAT is going to get us the extra speed.