Brschmid,

The most efficient setup would be the engine running at peak power (higher RPM) and the prop running at peak efficiency (larger diameter and lower RPM).

This is true for full scale as well (turbo props are a great example as they are geared). Recip engine full scales match engine RPM and propeller RPM fairly well (and add adjustable pitch props in many cases).

I'm guessing the reason it isn't done often for models is, as everyone has said, for reasons of cost and reliability. I'd also add that the need for efficiency in models isn't as great. Thrust to weight ratio is typically much higher than for full scale airplanes and also no one really cares (too much) about fuel economy, which is a big factor in full scale (sometimes).