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So you're doing +600 in addition to the EVGA overclock in bios or including? Mine seems to throw cuda 30 errors anytime I approach +415 in the precision x app. Still have only been able to get 315 on one of my evga ftw cards. The MSI gamer cards seem to get to 298 before giving the same error in cudaminer. Seem like totally stable overclocks besides those errors happening eventually.
It's luck of the draw, oc wise, but it also depends on the types of memory chips used by the manufacturer.
We can say on average 300kh/s per 750ti after ocing it, which isn't half bad for 70-80w.
Now, is that smarter than mining yacoin with them? Probably not the case, keeping in mind all of them seem to do over 3.2kh/s and draw 18-20w whilst doing so.