Many of these cards benefit from downclocking the memory speed. This helps reduced the total heat produced by the card, and for reasons I don't technically understand, this can positively effect hashrate. When I downclocked the memory on my ati 5830 XOC's to 300, and later 200, I noticed a useful temperature drop, and a slight increase in hashing speed.
I have never had increased hashrate from downclocking, but the heat is lower and that is what you need. You do not want to kill your cards VRM (and therefore in essence you whole card). I also use MSI afterburner to lower the voltage which makes a big difference in the heat as well.