It probably would but, What ever you would gain in response would probably be lost in cornering stability. The r&d guys put a lot of time finding that perfect balance for each bike and IMHO changing width either way on any street bike (unless it's a purpose built bike ie drag bike) would hurt the overall performanceThe rear wheel width of the fz6r is 4.5", which means the maximum size you can put on is 170. 180 is definitely not recommended by tyre manufacturers and would result in the tyre being overly deformed. The FZ6, R6 etc run rear wheel 5.5" wide.
I actually wonder if going down to 150, would sharpen the steering up a fair bit......
Running a slighly narrower rear tyre would not affect cornering stability, infact arguably the slighly smaller tread would experience smaller variation in forces and would be more predictable.It probably would but, What ever you would gain in response would probably be lost in cornering stability. The r&d guys put a lot of time finding that perfect balance for each bike and IMHO changing width either way on any street bike (unless it's a purpose built bike ie drag bike) would hurt the overall performance