I can't speak for FPE, but advertised RPMs on any particular prop/fuel combo are seldom obtained in real life. I've found that horsepower ratings are usually useless because of the size prop and RPM they use to obtain them. Experiment with the prop until you get the performance you want, despite what the manufacturer says. For example, a Zenoah G-23 isn't supposed to turn an 18-6 prop very well, but on my 1/4 Fokker Dr.1, it works great!

Dr.1