Problem with more ships in a task group is lower FPS. For a H2H Midway Map I made; I used 11 destroyers, 5 cruisers and 4 carriers for the Japanese fleet. I got an average FPS of 25-30 when flying alone through that, and when I changed countries the amount of artillary it put up caused FPS to drop to 15-25.
Now add 40 planes attacking/defending that, catching fire, ships catching fire, explosions, ships sinking, tracers and skins. I would be lucky to keep 15 FPS. (I have an average system, I'm sure a lot of people with better video cards, etc. would have no problem with FPS)
I think the hard part of adding a Battleship is keeping polycount down. There would probably be as many polygons in a single Battleship as in 2 CV Task Groups in AH now.