This touches on one of the things I wonder about in the playlist management: all maps in a given playlist are configured similarly. Corvette and Overlook have the same time limits, while all maps in a MP playlist have the same load outs. I wish they would tailor the settings on a per-map basis.
Unearthed is indeed fantastic. In Score Attack, try Crashsite. Some of the most intense 10 minutes you can have in the entire game. On most of the maps the defense strategy is pretty one-dimensional, but on Unearthed, every time I play I figure out better and better ways to hold each location.
Multiplayer and Firefight use different game engines, and the map would have to be rebuilt as a MP map. Jeremiah on the Optimatch forums said converting one to the other would be like building a new map from scratch.