It's all physics. Hardware limitations are just that. If you are already getting artifacts on your GPU overclock settings, I would think applying more voltage would create more heat and instability, just an opinion.
I run my XFX GeForce GTX 260 Core 216 at 750 Mhz core 1160 Mhz memory bus 1617 Mhz Shader with 70% fan completely stable on stock volts and no heat issues. 18,250 = 3DMark06 score w/e8400 at 4 Ghz.