SGS1 which uses a similar Cortex A8 1GHz CPU and it was quite stable for most people up to 1.2GHz even with undervolting. The later US variants such as Epic/Fascinate etc could go up to 1.6GHz and some found it perfectly stable up to 1.5GHz. So based on the N9 using something similar, I can make some slightly-educated guesses.

- 1.2GHz should be a guaranteed stable clock frequency IMHO. Might even be considered a perfectly safe overclock as it may hardly put any extra stress on the CPU. It should also give a nice 10%+ boost to app loading times, web page rendering etc.
- 1.4GHz will be stable for some probably. Just guessing here but I reckon it's doable and if lucky, 100% stable for over 50% of people. I wil be very happy if I can achieve this.
- 1.5GHz+ is very uncertain. It all depends on how good the SoC used in the N9 is. But if it is possible and stable, it should give quite a noticeable improvement in performance.

I reckon N9 will show a much more noticeable difference once overclocked compared to N900 due to the huge boost in RAM. My N900 ran at 1.1GHz ALL the time for over a year never missing a beat and although it's definitely noticeable, the lack of RAM limits N900's capacity to multitask bigger apps smoothly. (even a few MicroB browser instances can make N900 laggy)

So someone please make this happen on the N9 like how it was done on the N900!!