I got into 6.5 Grendel with a CZ 527 bolt-action rifle for deer hunting. No regrets. Whether it is better or the more recently introduced 6 ARC, I couldn't say, but the 527 was never chambered in 6. Either would work in an AR-15, but they both perform much better with a longer barrel than most people prefer on an AR.
AR's also bring a lot of other potential criteria into the mix. Heavier projectiles are going to have a better terminal effect, but if the bulk and weight of a full loadout is a factor, then the trajectory of the bullets is going to arc more because larger cases to hold the powder for higher velocities would reduce magazine capacity and therefore increase magazine count and overall weight. If you're hunting and you only need about five cartridges, it doesn't matter.
300 BLK is the worst of both worlds. It's too small and light for subsonic, being beat even by 45 ACP. It's too heavy and with insufficient powder capacity for supersonic. Grendel is certainly a better choice for supersonic. For subsonic, we need some more innovation. 300 BLK works well because it uses the 5.56x45 case, magazines, etc., but like I wrote, it's too small and light to slow it down. The 45 is better, but it may lack the pressure to operate a gas action. Pistol cartridge carbines typically use blowback actions that don't just blow, they suck. Suppressed AR's need a large caliber cartridge that generates a lot of gas pressure. I don't like how the straightwall options headspace off the case mouth, so maybe 458 socom or 9x39.