I'd think that a flat first serve technique is the most efficient, would'nt you agree....for hitting a ball fast, under some control.
So why would you choose to use another technique, less pronation, for your twist serves?
Just some food for thought. I used to NOT pronate much for twist serves, and it's jump up around forehead heights on the good ones. Now I pronate as much as a flat first serve, and the ball SEEMS to be jumping a hair higher, but also has more up and out spin. More spin is good. Height appears to be influenced by how deep I arch the twist serve in....so maybe a consistency issue. I don't practice at all.