I need to switch a project over to Mega right now, and I do not have time to do power testing. Can someone tell me if MEGA will use similar amounts of current in sleep modes? Do the same low power library functions really work?
Yes, but double check with the datasheets if you want precise numbers and differences for the different modes of power down of the 2 chips.
Everything in code should work the same, yes, if not bugs can be reported and will be fixes as soon as possible.