Just an update, overclocked the 460's to 800/1600/1800 with stock fan and voltage and it's now reporting 395W for combined 26K PPD production.
So a 10.4% overclock on both 460's consumes just under 4% more power from the wall plug and gives about 2K more PPD.
Thought I'd add.. the temps have really not varied much.. 2-3C at most