If you ever need to unroot, in case you have to send the phone in for warranty service or whatever, find your firmware in this thread
or if you can't find anything specific to your carrier use the most recent one for your country from here
and flash in Odin.
Do a factory reset just to make sure there's no apps laying around on the phone that might tip off whomever repairs it (Samsung) that you've been running non-official firmware on it, and get a jig from eBay to reset the flash counter (do a search for "Samsung Galaxy S2 jig", costs a couple of bucks).
Not sure about the MW thing as I've never played it, hopefully someone else might come along and offer a hand (Maybe change the title of your thread ?).
Originally Posted by redfreak
Yeap. Done. Had issues with Modern Combat 3 lagging in Singleplayer. Hmm, btw, any guides to unroot? Just in case I needa claim warranty