Holly, you don't get many more shots from a regged MPR, maybe 10 more.
The velocity curve is already very flat on the MPR. Remember the reg takes some space in the cilinder and reduces the usable air volume. The only advantage of the reg (I've got an Audrius in my S400) is that you loose the velocity spread/curve you normally have. But to get a good shot count from the regged rifle you also need to tune the hammerspring, the firing pot, valve spring and for best results also the hammer. A lot of work for a few more shots. Especially for the MPR I would not do it. You risk a reg failure or a leak on an otherwise perfectly reliable rifle. The FAC air cilinder is the better option if you can handle the extra weight at the front.