There is very little difference in engine performance between 15 and 20% fuels - maybe 100 rpm or so. The needle setting on your engine will be more affected by variations in conditions than this minor fuel change. I've run fuels with nitro ranging from 5% to 70% for over 40 years, and except for a few engines that are set up for FAI type fuels, most model engines are pretty tollerent of minor variations in nitro content. Oil content is a different matter, and having the right prop and needle setting is most important to keeping an engine running well.

That and avoiding dirt.