It kind of depends on what Mats is referring to when he says "DP could hit it harder with proper technique."
DP hits a bit flatter than a lot of the men, though John Yandell says he's still getting a lot of ts on his forehand - so he's not hitting flat. Does Mat's mean that DP could hit harder from more positions, especially if the ball's lower and closer to the net, and needs the ts to keep it in, than he can/does with his current fh technique? I can't say personally because I haven't studied DP's fh enough to say. I know for me and my little peon strokes that one huge advantage of the modern fh is that I can swing with very high rhs from almost anywhere on the court and just vary the amount of spin vs hitting through the ball based on the situation and how brave I'm feeling.
Generally DP hits it really hard, so I don't see how generally DP is wasting a lot of potential pace.
When I've looked at slomo of his fh he doesn't get into ptd as much as some other guys. This links to 5263's comment on leading with the edge of the racquet and the waiter tray comments from others. His motion is a little different generally, and not as pretty as a guy like Federer. But he hits that fh hard. I don't see you can seriously say DP is has a lot more power potential in that shot and still keep it inside the earth's gravity well.