Original reference GTX 680 has Power Target max 130%, max voltage is 1.17V and GPU Boost is still active. But some upcoming non-reference cards has more better features! Power Target 150%+, GPU Boost OFF and Higher Voltage!
On GeForce GTX 580 does not increase voltage through I2C commands to Buck controller, but through the BIOS. The same applies to the GeForce GTX 680, for higher voltage is necessary new or modified BIOS. The best thing there is possibility to off the GPU Boost, it also can be done in the BIOS. GPU Boost value is simply set as Base clock value... this possibility, some non-reference cards will have!