Absolute, serious hogwash. The 300BO's major claim to fame is subsonic, heavy bullet, suppressed. That is it. It retains the same case as the 5.56, so can only generate roughly equivelant energies.
The 6.5 Grendel, on the other hand, utilizes a larger case and a much more ballistically superior projectile to achieve higher energies at ALL ranges, and can remain supersonic out past 1000 yards, and still within the AR-15 platform.
Knowing a little about the platform, I would recommend to anyone interested in achieving the most possible out of thier AR-15, to do some research on the 6.5 Grendel varient. Once you know the facts, the choice is clear.
I have flirted with that caliber but I do not relish the idea of maintaining another library of cartridges to feed it. You are correct. The BLK really shines in a SBR, canned variant with subsonic rounds. But, damn... those rounds get pricey especially if one goes with Hornady..
I have also been intrigued with the creedmore but... again, I hate the idea of buying another 1,000 rounds to keep in stock.