I find the gearing on the Monster 1200 a bit long. Since it's just about time for me to change the chain, I'm taking the opportunity to up the size of the rear sprocket from 41T to 43T (+2R) as well. I purchased the OEM 43T rear sprocket for the Diavel which fits the Monster 1200. If you've done a similar change, I'd be interested in reading your comments.
1st, 2nd and 3rd is not difficult, 4th and 5th over crests is fun but speeds are getting silly.. lol!
I did this on mine, changed the read sprocket with a Supersprox Stealth 43T which is 400g lighter , front sprocket with a 14T and kept the 108 links oem chain. Now the idle speed is about 11km/h which is much more manageable in parking lots.