Ok for those who are interested I've managed to get 1241MHz out of the GPU by increasing the voltage to 1.2v.
In order to do this I had to use the Keplar bios tweak program that Praxis mentioned, whilst having NVFlash in the same directory
At first I ramped up the voltage to just above 1.175 to see what it would do but it often lead to screen freezes as the actual voltage as going quite a bit higher. After raising the voltage again (just in case) and many restarts I decided to set the maximum voltage in the bios to 1.175, giving me a solid 1.200v when in game.
This meant once I'd tweaked my settings in Asus's util, I was able to gradually ramp it up to an offset of +150 which gave me a very stable 1241MHz, at no point does the voltage or frequency deviate from this during benching or gameplay like it did frequently when I gave the chip too much voltage or power, although now my fans are running at around 85% just to keep the chip below 70. Thankfully Asus's design doesn't make this unpleasant.
All the games I've tested it with do get improvements without artifacting, but i guess I will have to see if it is completely stable as I play games the whole way through.
On the other hand the memory latency speed factor has increased 5 points from 13.3 to 13.8, equaling some of the worst in my original chart so one day I may go back to the and start testing the memory all over again.
Saying that, Arkham City gave me the best result of 61FPS the first time I benched it, usually I have to bench it a few times as it sometimes it starts off at 37FPS instead of the 77 I know it's capable of. Also the minimum framerate is often skewed by the frames between scenes when the screen goes black. It will often leave a scene with a decent score and go into the next one with a much lower minimum framerate which lowered the average by a few fps so although it is not the most ideal benchmark, it still represents how a game with a lot of features has to balance them.
Edit: well that was short lived, as soon as I fired up Ungine Heaven and it got to the heavy tessellated dragon scene, it just froze again.
I tried down-clocking and it seemed happy at 1202, which also means I can lower the voltage to 1.187, which is good in a way as my card will probably last longer and it will cost less in the long run to run it. The maximum frequency I can get at this voltage is 1202 which I'm still happy with, it doesn't seem to deviate from this.