Gary,
if the only mod to your bike are the Mistrals, you don't have to worry. From what I've read a free flowing exhaust only doesn't need a changed map on your Eprom.
However, when you start extending your mods (X-over, K&N filter, etc.) you can opt for a PowerCommander or have the Eprom "flashed" (written over by a custom map) by someone who knows what he's doing.
If you want more info on the PowerCommander, just do a forum Search