Stick with the 170, the bike will turn better. It is pretty common not to use all the front on the 900s, and many other bikes for that matter. Tonly time I use all my front is on the track but I push pretty hard on the front. I use pilot powers on my 900FE and use a 170. The 900 is a slow turning bike so much so that I use a 60 front profile versus the stock 70. Regrding fit, a 180 Mich will clear the chain, but a Dunlop might be a problem. Besides, I dont like a chain running that close to the tire with it throwing a little chain wax on to that outside edge.