If you want better performance, change the tire to a different brand and model of the same size specs. Do not change the tire sizes. It will put you at risk since there are only two tires on the ground instead of 4. I use Michelin Pilot Road 4s. They are phenomenal tires and would recommend them. I am eyeing the Perelli Angel GTs in the season after this. Still have lots of tread on my tires from 5k of use.I run the stock tires at currently run 120 70 17 and 160 60 17.
If I change to 120 60 17 and 150 60 17 will it change to a lower center of gravity feel? Will giving the bike less sidewall make it a bit less top heavy. My feeling is no but its worth a thought.