The answer, It depends... My 560 I was able to go from ~920Mhz to 990MHz from the antec 620 mod and removing the IHS, which you don't have one. And then I was only limited by the maximum voltage the card would let me go. Bios mod didn't work... Even tho that's only a 70Mhz boost, its still 8% higher clock which is great! I found my card would crash at about 74C. Then once I did the mod, I could crank the voltage and MHz to its max and the card wouldn't even flinch at its max voltage. My 580 on the other hand isn't overclocking well right now. Not sure why but for some reason I cant get it past 935MHz and it tops out at 55C on the 620.
You could give watercooling a try. However, it might, it may not, give you more OC. It just depends. But what it will do is most likely add to the life of your card! That's forsure!