Why does "The 6.5 PRC make the most sense"? He already has a 6.5 CM.
.264"
.284"
.308"
A good 7mm fills that gap between .264" and .308" very nicely. Plus allows for a much heavier range of high BC bullets to be shot if desired. The 7SS was designed to fit in a true SA while leaving room to seat the bullet out to allow room for the heavier long bullets to not take up too much case capacity, the 40* shoulder is very efficient as well.
Lancetkenyon - I'm not familiar with the 7SS but it sounds great. is the 7SS a SAAMI spec'd cartridge?