I personally have never overclocked laptop hardware b4, only PC hardware. I have always been a bit worried about overclocking laptops due to heat build up, laptops cannot get rid of heat as well as a desktop, so overheating is always a worry.
With regards to your overclock, I'm sure you can go higher but be very watchfull of your system temps. I use NTune to overclock NVidia GPU's (Dekstop), what are you using? Not sure how much further you can push that card but make sure everytime you overclock, test for stability. I use Furmark or a heavy graphic game for a couple of hours. I would suggest carrying on overclocking, keep testing every 15MHz and watch your temps.
Keep trying until artifacts are shown. I assume that with every change you are running tests for stability and temperature. Install ATI Tool, 3Dmark, Furmark for testing. And at the end try playing games to see the result.
p.s As JRock247 pointed out be very careful, because you are OC-ing a laptop