...you can add more pistons and keep the same displacement. I guess I was assuming that you would make the pistons smaller when you add more.

So like going from a 41/44 to a 31/35/38 for example.

You are right about leverege on the larger disc, no argument there. Though unless you're making some drastic change (100mm for example), something like going from 321mm to 332mm will be pretty marginal in it's affect on feel/bias. Now, i said marginal, not non-existent.