Ethereum mining is entirely memory bound rather than CPU bound.
What that means is that the stream processors, while necessary for some caclulations, are generally not the bottleneck. Memory bandwidth is.
Because of this overclocking the RAM on the graphics card is what will increase performance, not the GPU itself. You can underclock the GPU or restrict the power to the GPU, to a point, and see only minor drop in hash rate. This can give you a performance boost and power saving at the same time.
Read more at Small tweaks to improve hash rate and reduce power.
From the Ethereum github page:
Ethash PoW is memory hard, making it basically ASIC resistant.
...
It is designed to hash a fast verifiability time within a slow CPU-only environment, yet provide vast speed-ups for mining when provided with a large amount of memory with high-bandwidth.
This is simply rephrasing what I've said. You don't need a massive amount of CPU power but having a large quantity of high bandwidth memory, as you would find on a graphics card, is what will really be the key factor for hash rate.