I'm no expert and I might be wrong, but I doubt it would increase your battery performance (to the contrary, it might increase it...).

The reasoning behind that is that no matter what resolution you have, the phone's display still consists of 800x480 pixels (even at a lower display resolution, the screen hardware is the same). Also, when factoring the processing power needed to render the lower resolution (the phone will need to compute the native 800x480 pixels into 400x240 or whatever), you'd probably be worse off. Not to mention application incompatiblities...

Just my 2 cents. :-)