I"m no scientist, but rather a dumb artist.
I assume foot back means platform stance?
Since I played back in the late '70's, I use pinpoint to get to net, with my momentum already started. I get to net late.
Edberg, Rafter, and Cash get to net early and deep, as did McEnroe. They all had slow serves compared to their peers.
Dr.Ivo, Roddick, and Milos have fast serves. They seldom get within 2' of their service line by the time they are forced to volley...talking first flat serves.
Now, is a moving start with feet together quicker than a static start with feet apart?
For sure, a wide reciever would choose feet apart, but he's required to be static stopped before the ball is hiked.
A base stealer in baseball needs BOTH to insure a stolen base.