More voltage to the GPU, higher power target. I am assuming when you refer to 1420 MHz and wanting to achieve 1600+ MHz you are meaning boost clock? The card's max boost clock is set in the BIOS of the card. You can use a modified BIOS to increase the max voltage to the GPU and max boost clock and also the max power target of the card to get a higher over clock but again, no guaranty it will be stable without being able to bring the GPU internal temp down to or below ambient temps. Each chip is unique to itself. No two are identically the same. Just the nature of the chip's manufacturing process. I used to work for a company in IT that made the wafer manufacturing/handling equipment for the chip fabs around the world. Very complex and scientific process. I am still amazed how they can produce such complicated ICs with any kind of good yields off the silicon wafers. Remember though, more voltage will increase the temps on your card. Two 980ti's in SLI should be fine on a 8x by 8x PCI-e lane configuration. There are no video cards out right now that can fully saturate a 8x PCI-e bus. You might see a couple of FPS drop on that kind of setup but nothing that will have any tangible effect on performance of the cards.