I use a constant pull and string in the mid 50s. I use cofocus and right now have Supra gut in there. I string both at 56#s since cofocus is real elastic and drops tension. I end up with a stringbed in the low 50s, which sounds like a trampoline, but I have learned to control that and use it to my advantage.
I talked with a teaching pro about this, and he was saying a ton of pros use this setup now because of how relaxed and flowing you can hit the ball. That is what I needed to cut down on UEs because I tend to swing too hard. There is a difference between swinging too hard and racquet head speed though. He was also saying that this is a perfect way to hit really hard and heavy shots and save your arm in the process.
I think it also helps to have a heavier racquet to give you some power too. I went with the PDR with some lead for that reason.