RE: 4.0GHz OC
With a good aftermarket cooler you can get 4.4-4.6GHz out of an FX6300 provided your motherboard has good VRUs/power phase.
RE: Bottleneck
I had 2 x 7950's in crossfire on a Phenom II 1090T @ 4.0GHz playing Crysis 3. That x6 CPU was 'JUST' enough to push that dual GPU setup to 95-99% on each card. All 6 cores on the CPU were at 90% or so. This was on a Gigabyte 990FXA-UD3 so I'm thinking the dual PCIe @ x16 support helped. Granted, your FX6300 is different, but should perform similarly on a good board if cooled and clocked to full potential.