From what I have tested and from what I seen. Not one card perform at it best when clocked to the max.
There is a sweet spot you need to find when it comes to the overclock.
But it seems like most people belive that since the numbers are higher it has to be better. That is not a fact.
Start by adding +100 Mhz until you see your FPS or points go down (depends on what program you are running to test / stress the card)
Once you see performance decrease dial it back 50 Mhz. Then play around with + - 10 Mhz until you find the sweet spot.
Same goes for the GPU core.
EDIT: Oh and welcome to TomsHardware by the way