You are right on power output on some of the 4-c engines. The YS engines in particular are outstanding for power to weight. The YS110 is an animal.
However, most 4-c engines are RPM limited due to the valve springs. Most have to be propped to run under 10K rpm on the ground. YS 110s have been known to safely unload up just over 11K rpm in the air. The Jett BSE-120 is the most equivalent engine, designed for lower rpm and big props. Same props, same rpm range. Easily turns a 16x8 near 10K rpm. And although it should not really be run over 12K rpm, it is not physically rpm limited by springs and stuff.
The Jett 90, for example, is the same size and weight as the 120, but by design is not as rpm limited. The rpm key here is the muffler range (And that can be changed). Typically, the 90L runs between 13K and 15K rpm. Prop selection is made by that. Tuned differently, I have had a 90L up to 18K rpm in a ducted fan setup. Engine didnt care (in fact, it loved it).
For speed, there is nothing like a small diameter prop (less disk area, less drag), high pitch, and high rpm.... as long as the plane will go along for the ride
