The problem is maximum core clock is not important, what's important is how often your card maintains the max. Here, check this out:
You see how the card is dropping super low, down to sub-1200mhz multiple times at the start of that 3dMark11 run? That's because that portion of the benchmark was drawing so much power that it would spike my power percent usage (you see it's at 117%, the max) so the card was forced to throttle down to 1167mhz. However, if you look at the graph, you can see my max core clock was still 1257, but it didn't constantly run at that speed.
Now look at this one:
By increasing my power percent by 5%, it allowed the card to stay at a relatively constant core speed (only dipped once to 1244). Despite having the same maximum core speed of 1257, this run scored 15% higher than the previous one where my Kepler boost was getting constantly throttled.
If you're already at the maximum power percent, the only way to stop the core speed from throttling is to reduce the core offset a bit. It might reduce your maximum core speed, but it will give you increased performance because the card will run at a constantly higher core speed with less drops.