yes you can overclock a factory overclocked GPU, and sounds like all guys are using MSI AfterBurner to overclock their nVidia GPUs.
Max Overclock ?? you can go up to 900 MHz and above if you've a water cooling.
but i would suggest you try your own, bump up Core clock a little bit by (30 MHz), shaders by the same amount, Memory Clock by 100 MHz and check stability (run games, bench test,... ) if everything is okay and no crashes, try increasing all those by (xx MHZ) and do a stability test. if everything is okay try to go further.
Note 1 : don't forget to check temps in each overclock you do.
Note 2 : if instability happens increase GPU voltage a little bit, 1.088V is your max, don't recommend you go above that and you can ahieve above 850 MHz Core Clock, 1750 Shaders, 2000 Memory Clock with that voltages.