This was demonstrated by overclocking legend Shamino, who managed to push an Asus card to a whopping 1519MHz.
Of course, this really isn't something you should or could do in the comfort of your own home, as it takes quite a bit of LN2 and a bit of tinkering with the voltages. The core was juiced up to 1.5v with 2.12vdimm and 1.35vpll.
You can check out the gory details here.