No matter what, overclocking is a gamble and the only way to guarantee a higher clock-rate is to buy the card that comes at a higher clock-rate. You very well might buy the lowest-end version of the card and it could be one of the best overclocking cards out there, likewise, you might get the highest-end version and you could barely squeeze out a few extra megahertz.
It's generally accepted that the high-end cards like the FTW, Classified, G1 Gaming, etc... cards are the most selectively binned cards (which is why the are clocked the highest out of the box) and therefore have the best headroom but it's still ultimately going to come down to the luck of the silicone lottery. There's no guarantee that you'll be able to go higher than the stock settings, only the guarantee that you'll be able to use the stock settings.
Also, I don't know if this is the same for the GTX 980 but I know for a fact that the GTX 970 ACX2.0 FTW uses a slightly different heatsink than the ACX2.0 SC and ACX2.0 SSC.