If I'm not mistaken with the SH4 Mission Editor you can play a mission anywhere you want as Uboat or US Navy.
Confirmed that you can do U-boats in the expansion in much mire then the Pacific. And I get to kill Gerrys in the Pacific and well as Japs
I didn't really care for the new jap/german planes that narc on my position when above water.
Germans can also have surface vessel and air support.
The Expansion is definitely worth it.