Very interesting. Some time in this past year, I read about this MPC in one of the "special weapons" magazines. Something along the lines of "time to upgrade from the 223 now that it sucks" and there were three different cartridges, one of them being this one. I forget the other cartridges, but the idea was exactly what you mention - more "power" with a simple barrel swap. Interesting that they showed it still worked in the .223 SAW links.
There is also a 6.8 conversion that is simply a necked-up .223 with a 6.8. I think it is referred to as the 6.8-223.