Hmm, I think the system itself should be intelligent enough to do this. Why should a user have to take the burden to decide this?
Can you come up with an explicit usecase for "save battery -> 400MHz, low light on the screen" that's not covered by the current N810 that dims the screen after some time anyway?
Why would you want to not use the complete power for "normal" performance? 4 days of battery life in stand-by mode not enough? :-)