I don't see how 180/55 would turn in quicker than 190/55 as the 180 would be stretched wider to turn the 6" rim and therefore flatter. Both if They are different tyres there is no valid comparison as some.manufacturers 180s are wider than others 190s
So after some research, I can't find any source that suggests that a 180/55 is "stretched" on a 6in rim. If you do have a source for that I'd be interested to see it. There is a thread that goes into some detail on this on the Gixxer forum http://www.gixxer.com/forums/showpos...9&postcount=13
It is definitely not meant to be a definitive source, but is the info that a member took the time to gather to answer these questions, and did a fairly good job in analysis.
The 180/55 and 190/55 have almost identical profiles, with the 190 being only slightly taller and having slightly more meat. It seems mostly to be a rider preference for situation, many riders choose each for different situations and effects.
The main gain I can see is the unsprung weight of a 180 is considerably less, which theoretically could lead to slightly better acceleration, breaking, and turn in in certain situations. However that could be matched by the 190s improved contact patch in other situations.
When I have tried both on this bike it has always been with Metzeler m7rr. I did like the feel of the 180 and think it is totally fine to use if that is what you are comfortable with, but ultimately I am back on a 190 now.