I had a 180/55 on my M900Sie, and just switched to a 170/60. Couldn't be happier. It fits the 5.5" rim better, it's lighter, cheaper, and it accelerates/handles a little better - IMHO. I put a 180/55 on my 996 for the same reason - it works better than the stock 190/50 for me.

The wider tire is for looks only. If you are dead set on this upgrade, go to ebay and pick up a 5.5" rim from an M900 and go with a 180/55. A 190/50 does not fit.

Also, Rumor has it that Dunlops are wider than other brands for the same size tire.