I did a lot of testing today overclocking and underclocking my gtx 970. I have an rvz01, so thermals are quite a big problem. I can get 1480 MHz boost and +350 MHz on the memory at stock voltages and 87% fanspeed at 84°C. However, when I limit the fanspeed to 54%, I'm still able to get 1202 MHz (which is still above the core clock) and +350 MHz on the memory, this is also at 84°C. (
here is the fan 'curve')
The reduced clockspeed may seem pretty bad (±20%), however, the performance decrease was only about 12%, meanwhile, the noise went from unbearable even with headphones on to very acceptable.
I don't know how to decrease voltages on my GPU, MSI afterburner only allows me to increase it, but even without undervolting, I managed to significantly improve my gaming experience.
(BTW, both test configurations where at 85% power limit.)
I guess overclocking in an itx system really doesn't make a ton of sense; you need so much better cooling to squeeze out a small amount of performance. (And I didn't even increase the voltage, doing so probably yields even smaller diminishing returns.)