65 Receiver Set with Polymer mags from PF (HOPEFULLY)

[stanc]

It seems the ideal mag would be so wide that we'd need a magwell wider than any current AR, correct?

Correct. For a 6.5 Grendel PMAG, the mag well would have to be wider even than the LWRC SIX8 mag well.

So then would the ideal mag be so wide that it'd affect reliable feeding with the center line of the ammo being shifted so far to the outside?

I don't see why. AFAIK, the stainless steel mags currently used achieve proper stacking. The centerline of the cartridges in a "SIX5" PMAG would not change; only the external width of the magazine would.

Then could what your suggesting have a the inverse problem?

Yes, indeed. Doing as he suggests would result in shifting the centerline of the rounds inward.