Pi calculates approximately 31% faster at 800 MHz compared to 600, which is very close to what one would expect

So far my device has been stable at 800 MHz, and the boost given for any CPU-bound task is very nice. After getting used to this it would probably suck to go back to 600

One has to remember though that even if the device runs stable, it's lifetime could get seriously reduced due to increased thermal stress. So those users who were worrying about their N900s getting obsolete probably don't need to apply...