Hi Markus,
I have a 2001 J2 and had to fit a set of tyres at the Isle of Man TT. My only option due to limited avaiability was a 170, it does not look so good but man does the bike turn fast on it. It may be the change from Dunlop to Metzler too but the original Dunlop's were well rounded and scrubbed, so squaring off was not an issue.
I think a 190 would slow the turn in too much, but it would look the business. I am now caught between the 170/180 option for my next change but when my sliders surfing the local roundabout the 170 wins, when it's parked up the 180 wins.
Have fun,
Hugh.[8D]