Actually, the memory bandwidth is what you need. You can push that clock up to 1000Mhz, but with no more memory bandwidth, you get no performance gain. I have done it with my GTX 460 1gb and experimented many times. Other people have said the same thing.
Adding card voltage doesn't add voltage the memory, which is the limiting factor. You have to push that memory as far as possible and tune the core/shaders until the bandwidth isn't enough being provided by the memory.
Anything is good on stock voltages, I mean, you are getting more performance!
Add some voltage and see how far it will go. I run Furmark for 30 minutes to an hour checking on it periodically for artifacts.