As far as my experience goes, If you drop the width on the rear you flatten the contact making the initial turn harder with a very abrupt lean on the outside. it would be better to swap rims with a C model 9, drop the front, raise the rear, or lower the profile on the front tire. (not width)
It's fine with a 180. Check out my findings about my new M1's in this thread
To be fair The Fruit Bat is right about the tire shape if you try to go too far from stock but the 5.5 inch rear on our 9's is within the suggested range for 180's according to the Dunlop fitment guide. Of course this may all change for specific tires. But I can offer you a hearty thumbs up for the M1's with a 180 rear.
From (my) roadracing experience, the 190 offers a little better traction when gassing it out of slower corners. The 180 made my bike turn a bit faster and a lot easier. I chose to run the 180 so I could get the bike in on the brakes quicker for passing and just gave another 10% attention to throttle control. It also helped me not get tired as fast when endurance racing.