Not sure, don't have a means to measure it. Should be pretty low tho, the highest voltage running card is 1.093, the rest are spread between 1.050 and 1.070 and quite cool even in the hot weather around, don't reach 70. My point is something waffle said: cranking intensity up gets you stales, because the card has no time to submit stuff, on the other hand, having the same speed achieved with lower intensity makes the shares gets submitted very fast, thus the low rejects. Also observation between the 2 configs - one makes even the mouse lag, the other - you can freely watch movies and can forget there is even mining going on.

Edit: just checked the other 3 rigs with 280x on them - the highest reject is 0.27%, the lowest - 0.07%