-I would give all call in tanks a much higher CP requirement.
Shifting mediums to 10-11 Cp's and the Heavy's to 14-15 Cp's
-Give the call in a massive cool down (I'm talking 10-15 minutes or so)
Increasing CP's is the thing I would agree with. Call ins should supplement your vehicles not soleley consist out of it in my ideal world
But such a long cool down seems unneseccary