I've had outstanding luck with the ADG brass in 300. We've shot several different boxes of ADG in 6.5 with the same pretty good groups you mentioned. I'd be surprised if the brass was hurting the effort.
I need to save some Hornady brass from a good shooting load to test out.
As far as 10 round groups.. although statistically superior; I find them impossible to shoot well all the way through. Even on the .223 bolt gun, I start introducing shooter error. And 300 WM with A-Tip would be way too expensive.