The 300 BO is great for doing whatever a 7.62x39 can do as well, not just subsonic/suppressed stuff.
If you really want more power in an AR platform, the 308 is a pretty sensible way to go, and doesn't have to be that much heavier. I have a pair of 223 and 308 ARs set up almost identicaly (SS15 handguard, same Leup scopes, Magpul stocks, etc) except for barrel length (18" 308, 16" 223), the 308 is right about 1 lb heavier. It's also slightly larger in most dimensions, but not unmanageable.
