Each version of Android has found ways to increase efficiency of operation and give better battery life. I expect that ICS, especially knowing that it uses both processor cores in a way that GB does not, should help with battery life. We'll only know when it ships, though.
The amount stored in ram does little to battery life. How much of the cpu is used effects battery life much more. If ICS is programed to use the cpu more efficient than battery life should be improved even if it uses all ram. Like a previous poster said time will tell.
I wouldn't expect an appreciable difference between OS versions. The two biggest drawers of power on these phones are the screen and radio. Neither are impacted in a big way by the OS. If we're lucky battery life will increase going to ICS but the best you could hope for is a few percentage points. Look no further than the galaxy nexus. It's not setting any battery life records. The razr maxx is the best we can hope for in the next generation devices. Larger batteries bring longer life.
I have gotten slightly better battery life and the boot time is way better on the alpha version of ice cream sandwich for the bionic so far. Got to remembsr that I can't test 4G because 4G doesn't work yet but comparing Wi-Fi to Wi-Fi, Basically idling it doesn't use as much battery as gingerbread does