Righty and normally hit with semi-western grip/windshield wiper swing pattern - To hit a flat forehand with pace, does one change the footwork (say stepping more forward into and through the shot more) or just change to an eastern grip? Change swing pattern help (hit more straight through)?
You dont want to change grips. Really, if a player wants to, they can hit flat, with a WW, or a combination of patterns with all the popular forehand grips. Grips choice will emphasize which is more natural and which is more likely.
For example, clearly the Western grip favors a WW pattern because the palm is under the handle. Most people know how to turn a door knob and therefore, since the palm is under the handle on the Western grip, the position of the palm allows for more leverage to imitate this "door knob" turning hand pattern.
The Eastern grip with the palm more behind the handle, favors a more Nike Swoosh type swing pattern. Still though, if I really wanted to, I could turn on the handle a bit to create a WW motion with an Eastern grip. It is not ideal for me to do so all the time but I can if I want to.
For you, I would not change grips just to change the trajectory of the ball. You are asking for trouble there. I would slow the ball feed down and utilize a swing pattern that goes a foot below the ball and goes up and out towards the target. I would try to have the ball clear about 4ft high and work on it.
Basically, you want to swing in a swing plane that is rising gradually and go through the ball. Hitting flat will still need some topspin to bring it down in the court.