First, glad the templates are helping you.
And to answer your question, I have done a mission like that before but don't really have a vanilla template ready.. I'll start on one tomorrow.
Performance will obviously depend on your computer but in the end, Altis is a massive island and to properly populate it will be taxing. Arma update 1.44 has significantly improved MP performance so I actually really want to revisit this concept.
As for one commander being too strong, I guess I'll see for myself what's going on when I make the template. My first guess would be to force a certain amount of groups per side to even it out so one side doesn't get too many armor/mechanized groups.
If all goes well maybe I can have template for this up tomorrow, for now im off to bed.. school in 5 hours :(