Yes, that's part of the problem with the proliferation of 6mm options. Most new ones are pushing higher velocities and powder capacities. Then you have all the benchrest options to throw in the mix as well.
I guess the real question is what can I kill with a 6mm at 3300FPS~3500FPS that I can't at ~3000FPS? At what cost (weight, recoil, barrel life, etc.)?
Then there is the issue of the once a lifetime Mountain Goat hunt that may benefit from something like a 6-284Win. If a 243Win will take Pronghorn, why wouldn't something a bit slower like the 6XC? I could always take my heavy 25-06 if the shot requires more distance capability. Coyotes and Deer will never notice the difference IMHO with any reasonable shot placement at realistic distances where I would hunt (East Texas and Central Oklahoma) with a lighter (poundage) rifle shooting a softer shooting cartridge. Heck, a 6mm Benchrest variant would probably do well inside 200 yards which is worst case for most of my shots.