Can a player determine what's in their powers best interest using these new map features, or will I still end up advancing the wrong junk?
Better now than before. These features are not new, but they have been adjusted to reflect the cost of overheads. So the profit figure shown on the map for an uncontrolled system is more frequently correct than it was in the past. You are going to find, though, that there are not profitable expansions available for most (any?) of the powers. Expansions at this point are going to be selected based on other criteria, and understanding that is something best done as part of a group, as Rubbernuke suggested.
If you want to be a lone wolf, I would definitely suggest against ever running preps. Powers are probably going to consolidate, which means that if you want to help, prep is a wasted activity. If you're interested in logistics and want to support a power, fortify instead. If you really want to support a power, get in touch with that power's community to follow the systems that they have identified as needing forts.
If you run 1400 tons into a 8000 trigger system that is 10% undermined, you're...kind of helping, but if the organized community decides not to fortify that, then you've basically wasted that tonnage.