I run a Shinko 180 on the stock rear wheel. I do mostly straight line riding via redline shifting and like to hang in the Vboost range. I regularly hit 105-113 going to work in the mornings. The bike has frame braces, Progressive shocks and a Super brace between the front forks. The front forks are dropped 1.5 inches. The 1st gen bike have a soft suspension to begin with, but my bike feels solid. I doubt going up one tire size on the rear is going to affect handling to a large extent. What is your riding style?
If you want to really improve the handling, go with wheels that will allow you to mount radial tires.