In my experience my EVO didn't get that poor of a battery life, the battery life was just ok, not great. I did do all of the battery tweaks too.
I now have an iPhone4 and I would say the batter life on that is um well amazing. Alot of things go into battery life such as the OS obviously, but its also the CPU too. I'm not sure which is more efficient A4 vs Snapdragon.
The A4 is most likely an 800mhz processor instead of 1ghz, probably to save battery life.