NV should have at least allowed AIBs to ship after market cards with beefier VRMs, cooling
and unlocked voltage. With just 1.212V, 680 seems to hit
1350-1400mhz. I don't understand why NV had to lock out the voltage completely. After allowing EVGA to release 850mhz GTX460 FTW edition, this seems almost backwards.
Perhaps NV is going to launch "K" series chips with unlocked voltages. Here you go for $50 more GTX680-K with fully unlocked voltage for overclocking -- GPU overclocking of the future, the Intel way. :hmm:
You are confusing overclocking headroom with GPU performance scaling via additional frequency. Kepler scales fine, but it has low overclocking headroom.
GTX680 operates at a minimum of 1058mhz, with most cards hitting 1110mhz. With overclocking a lot of cards max out at just 1230-1250mhz. Therefore, the GPU scaling should be commensurate with the frequency increase (which is 1250 / 1110 = 13%). In fact, a lot of times GTX680 goes above 1110mhz, which means with average overclocking, GTX680's performance should not grow by more than 10-13% (and benchmarks are showing this because it's exactly what's expected).
OTOH, Tahiti overclocks from 925mhz to let's say 1200mhz (or a 30% overclock). In other words, it's not scaling that's the problem with GTX680 or that HD7970 GPU has some magical GPU scaling, but that most of the overclocking headroom has been "eaten away" by dynamic GPU Boost on the 680. It's incorrect to state that GTX680 has poor overclocking scaling.
Poor GPU scaling with overclocking means if you increase GPU frequency by 30%, GPU performance only grows by a fraction of the GPU frequency increase. That is not the case with Kepler.