I just got a UDM last week, and while I haven't used it on a car yet, I did compare the power it has to my PC. With the same pad mounted on each it was obvious the UDM had a lot more power (or torque) than the PC. At speeds 4 through 6 on the PC I could stop the rotation of the pad with the palm of my hand, but the UDM was almost impossible to stop at speed 4....... I could slow the rotations down but not stop them in my very brief test.
Lack of power has always been my biggest complaint about the PC, but it looks like the UDM has addressed that problem quite well. I'll keep my PC as a back-up and probably just use it with a 3.5" backing plate and 4" pads. If the UDM keeps me from having to use my rotary as often I'll be quite pleased.