Well I have AMD Phenom 9950 on my desktop, it is running @2.6Ghz 1.3V stock. I can overclock it to 2.8Ghz by increasing multiplier, if I go further my PC will crash, so 8% increase with no voltage adjustment. I could put it to 1.45V and go as high as 3.2Ghz but temp. increase is just insane after every 0.05V you do. If you undervolt or increase frequency without increasing voltage there is no extra heat. Underclocing dramatically reduces heat - FACT. Do not forget that this is 140W X86 CPU, but it still is a CPU - even though different architecture. I see A LOT of potential in that A8, Apple's iPad got one @1Ghz, no prob there. I ran it @1Ghz the speed increase is amazing. That is 40% faster with stock voltage (Titans ULV Kernel)
EDIT:
When almost idle it runs undervolted at 250Mhz which equals less heat, when on high load it does things in shorter period of time and goes back to 250 therefore less time high voltage and frequency, IMHO overclocing even extends life of your N900.