To a degree, what you describe is normal but not full throttle at 20% throttle. Any butterfly type throttle will reach nearly full throttle somewhere around 3/4 travel with very little RPM gain above that. Most people set up a throttle curve using mechanical differential, radio throttle curves or a combination of both to achieve the desired throttle response at the desired stick position. We can address that later. I feel your current problem is probably caused by the needles not being set yet. You need to tune that engine for idle and full throttle operation as you are probably very rich at full throttle now causing low RPM at that throttle setting. Tune the engine and it will act more normal.