I've tried overclocking the GPU on my iMac and have had some success, but trying to clock it back up to what are supposedly the standard specs for that GPU caused crashes. I'm guessing that means they've been putting chips in that haven't necessarily passed the tests at higher GHz. A lot of CPUs and GPUs are thrown out because they don't make it through testing. If the requirements are lower then more can make it through and be sold at cheaper prices.
I haven't tried with my MacBook Pro at all... But would be interested to hear how anyone gets on. Would prefer someone else to blow up their laptop rather than me ;)