My N900 already becomes a little bit instable at 900Mhz (the one program that appears to be indicating instability best is MicroB for some reason). I haven't tried increasing the voltage though...it's a tradeoff between battery and performance. But I would guess there's a clear difference between older and newer devices in production. Mine is older (Nov. 2009) and is not as 'overclockable' like I would guess newer ones are.