jaka

Hi!
You are right speed... All R/C model airplanes should be trimmed out at full speed! The reason is like you describe.
You trim a modelplane so that it flies without any trim changes from let's say 30% of full speed up to 100%. This is easily done by changing the Cof G as far rearward as possible and at the same time reduce elevator throw, trim the plane in axial plane and checking the wings for wash-out.
This process adheres to all model airplanes!