At stock clock speeds, my FX6300 bottlenecked my 7870xt (Tahiti). With a mild overclock, 4.5 +Ghz it was fine.
3DMark Vantage
FX6300 @ stock - P20978 -
http://www.3dmark.com/3dmv/4649590
FX6300 @ 4,661Mhz -P24036 -
http://www.3dmark.com/3dmv/4643604
FX6300 @ 5,008Mhz - P25082 -
http://www.3dmark.com/3dmv/4649609
As you can see, the jump from stock to 4.66Ghz is quite large, the jump from 4.66 to 5Ghz is much smaller.