I am wondering if there is any disadvantage from a GPU performance perspective of using an older Apple Cinema Display (2010, 27" version) via Mini Display Port instead of a Thunderbolt Display. I am upgrading to a 2013 Mac Pro and am wondering if my older monitor will suffice. In other words, is there any performance disadvantage to be connected via Mini Display Port & USB as opposed to Thunderbolt or should I see the same playback, rendering & encoding times as a newer monitor.
No none at all. When you run a display off Thunderbolt it runs displayport mode anyway. The display does not effect render and encoding times. The CPU, system ram, and GPU's effect performance. The Display simply effects preview output.