What about calculating in the increase of hardware errors?
The overclock is actually more efficient:
300 = 12.0MH/s @ 473w (39.4 watts per MH)
328 = 13.5MH/s @ 512w (37.9 watts per MH)
360 = 15.5MH/s @ 568w (36.6 watts per MH)
Edit: Tests performed with an 850W 80+ Bronze PSU.
Did you do a side by side to compare efficiency? Make sure the OC is worth it
300 = 12.0MH/s @ 473w (39.4 watts per MH)
328 = 13.5MH/s @ 512w (37.9 watts per MH)
360 = 15.5MH/s @ 568w (36.6 watts per MH)
Edit: Tests performed with an 850W 80+ Bronze PSU.
Bumped my Black Widow clocks to 360 and I'm now getting 15.5MH/s. More HW errors and power consumption went up of course...
Did you do a side by side to compare efficiency? Make sure the OC is worth it