You don't need to add one just because you've added an exhaust if that's what you're asking. The fuelling on the 650 isn't too bad as standard, so only minor gains can be had. One gain is on off throttle smoothness and sorting out the surging when at a steady state cruise.
Personally I wouldn't bother with a power commander as the ECUs can be remapped anyway. Take it to a decent place and have a rolling road remap done. It will suit your bike better and no extra devices required, and no extra devices to go wrong.
I've recently had mine done and am really pleased with the results. More to come on that one in another thread though when I get the pre and post dyno run data back.