I think that the main reason that pylon racing .40s are not tuned to be optimized at these rpms is not so much the mechanical limits of the engines but because of the exponential nature of prop load. At 44K, you would probably have to use a 1/2A size prop on a .40 and the reduced propulsion efficiency would likely cancel the gains of the extra power.
Big props turning low rpms tend to have higher propulsion efficiencys than tiny props at super high rpm, hence the popularity of gear reduction for electrics. With glow engines, the best prop is a tradeoff between engine power and propulsion efficiency.