But if you can't upgrade your camera to the new XF, I don't quite understand the choice of H5D-50c or Leica S. Since you are happy with the P40+, wouldn't it be simpler and more economical to:
-get a 645DF+ while they are still available or
-keep your P40+, have the mount changed to H mount and get a H4x or H5x? (you'll still need to change your lenses, though)
There is another catch: if you want to use Capture One, then you must use a Phase back. Phase One is locking all other medium format backs from their software.
Last but not least, if you are not using Capture One, consider changing everything and only need a x1.3, 40 mpix back (as your P40+), there are plenty of really cheap options on the used market with that sensor. You could even get a Pentax 645D (same sensor...) and get pocket change for your present system. Or an H4D-40 for a little more if you need a central shutter.