Nice explanation Bob....Once a player starts playing specific position on every shot, there is only one right speed for the position they are playing. I think George's advice was not for those players; he was speaking to the players who tend to use four-length speed on many shots.
There are a few exceptions to this, such as straight shots that can be played with an entire range of speeds to achieve a precise position. The main exception for pro players is the game ball, and often they play that shot with their most comfortable speed, which is not pocket speed.
Sigel was a great example of this.... obj ball had no rotation entering the pocket.
The sound of his hit pocketing the ball always had that ''slap'' sound hitting the back of the pocket.