I've noticed that most vanilla GTX 285 cards maxes out at 1620 on the shaders, when using stock cooling..

Even though mine runs very cool (doesn't exceed 65c) at 756/1656, it has a tendency to lock up... At 756/1620, it's rock solid stable though..

But when you look at the GTX SSC, it is seemingly capable of running at up to 1700+ shader speed..

It's probably a voltage issue, wherein the SSC is using a a higher Vcore giving it the ability to hit higher clocks.