Why is the world would it cost you battery life?

You do realize the OMAP3 is significantly more efficient than the OMAP2, right? It consumes almost exactly the same amount of power as an OMAP2 for 2-3 times the performance. Which, of course, means that for any given task you'll be getting to idle 2-3 times more quickly on OMAP3 than when performing that same task on OMAP2, and that means your CPU will be spending significantly more time idle (and, thus, not consuming power).

With ARM CPUs, your goal is to get to idle as quickly as possible (race-to-idle) because when you're at idle, the CPU is off and not consuming power (in fact, the OMAP3 consumes even less power at idle than the OMAP2). So assuming power consumption per clock doesn't rise, you'll get better battery life from a faster CPU that spends more energy for shorter periods of time than a slower CPU that spends less energy for longer.