The task killers kill apps out of memory but by the nature of the OS those apps will reopen which ends up using MORE cpu cycles, that is what can kill the battery. Leave the apps open and in memory, it is ok for high ram usage on Android. Android has a very aggressive memory management tool and will kill apps when it needs them.
As for rooting, I am going to say no. It's not going to solve any of your "problems" because whether it's TouchWiz, Sense, CM, stock, Paranoid, etc there will be high usage.
As for task killers, like you said android's task manager is excellent and adding more only uses more ram (unfortunately). The idea I'm getting from forums and other resources id that you get more control when you root and I want the full experience before I upgrade later on in the year to the Galaxy Note series. I'm stuck with this one for the moment because of funds...lol