I think Afterburner shows possible VIDs for many different GPUs. For example my old 6950 TFIII showed 1.10 and 1.15 VID choices, and it was a 1.10V card, meaning higher leakage.
A high leakage card operates at lower voltage to balance the otherwise higher power draw and temps. It will also overclock higher than what a low leakage card would. The problem is though, cards usually have a limit of voltage increase, say +150mV for 6950, which means AB could overvolt a 1.15V card to 1.3V, but a 1.1V card would crash above 1.25V.
A high leakage card is what you want if you do extreme OC, and you can keep the card cool (H20,LN2 etc.). Your mileage will vary of course, not all low VID cards are good OC cards, and not all high VID cards OC bad. It's a part of product binning, they try to fit in as many chips as possible to a similar spec.
