DTech in 243WSSM. Maybe even Mike's new 6DTI.
Like it was mentioned, get the Oly lower so you can load the longer bullets in the new, longer magazine, and also be able to get enough powder behind them without using compressed loads.
I have a 6.5 Grendel, and while it will do it, it's not my first choice. It's going too slow out past 600 and so it is exposed to the wind too long. At least that's the reason I miss more than I hit out at the 600+ mark. My wife killed her antelope with it last year at 467 with a 123gr. AMax and it did the job well, but I think that's about the limit for an animal in my opinion. The bullet did perform well, and it exited the other side.
I'm really liking my DTech in 25WSSM right now.