Interesting. My experience is that ultralight props spin about 1000/1500 rpm faster than a warbird's. Ultralight motors and gear reductions are all over the map but seem to generally have a max of somewhere between 1600 and 2600 rpm with the typical installation being toward the higher end. The typical early Merlin would have a prop speed of about 1400 rpm max, later Merlins under 1300 rpm. Low prop rpm his important for a high speed aircraft because of the necessity for delaying supersonic prop tip speeds.