If you are interested to know does the OC & kernel-config impact to power consumption (current drawn from battery), you can do very simple measurements by yourself.

Battery voltage and current can be read easily, but use steady use conditions (i.e offline) and lock & load CPU. Then calculate power at different frequencies and loading conditions and compare to stock kernel.

I share your concern about lifetime of N900 when OC, but your modification to increase heat dissipation sound bit dangerous.

Most probably you'll get data corruption problems much earlier than actual CPU damage, as Titan said earlier. Junction temperature inside IC can be quite high and if you start to get close to that temperature then you'll feel it, as device will feel hot. Then you should really consider lower freqs or short pause to allow device to recover to normal temp.

ARM core power supply capacitors are designed to work at 600MHz reliable. Most probably device will work reliably also at higher frequencies, some evidence available as we have quite many running with OC N900. Faster the ARM run, faster the de-coupling caps must feed current to processor (= paracitic inductances and resitances must be smaller). This is one limiting factor for OC and reliable system. In practise it means that core voltage will have more high frequency ripple and voltage dips. If voltage will go too low it will cause resets or other unwanted issues for HW&SW.

PS. Let us be polite to each other. This is a great forum and I have gained a lot. Thank you all!