The boost goes up as a square of the RPM. double the RPM get 4 times the boost. Up to a point when the tips of the impeller reach speeds where there are shockwaves, the boost stops. (The tips can actually break the sound barrier).
Often a poor ground on the switch will mess with proper wiper operation. Check by clipping a ground jumper wier from the switch case to a known good ground. Then if it doesn't still work try adjusting the park position tab on the wiper motor itself.