If I'm right in assuming that US spec S2R 800s are EURO 2 (emission bla bla)(check if you have O2 sensor on the cat) - they are more forgiving and will "accept" slip ons without any other requirements than adjusting fuel/air. = no new ECU required.
If you also change to a cat-less midpipe, like the one from MonsterParts, you will probably have to do something like a Ducati Performance ECU or listen cautiously to what others have done with PowerCommander.
All I know is that the DP ECU makes everything work very well.
BTW1 don't expect more than a little performance gain, but a lot more/better music
BTW2 a friend has an unused brand new ECU for EURO3 S2R 800. He's selling it cheap.
EDIT: Marving will bolt right up. Takes a little work though.