I have had just a short time on a second gen, the original tires, I am not sure even what there were they were old hard and crappy. After listening to others chatting about tires... my first gen had Metzlers on it and they were fantastic tires .... for me...... but the majority said Venom's for second gen. That is the way I went, knowing now what I do, I am glad I did not go with Metzlers, I think they would have been too soft for the second gen. As for size of tire, I have a couple opinions that may P some off but my first feeling is that Yamaha spent a lot of time and money with engineers figuring out tire sizes for bikes, if you change tire size you may gain a bit on slow speed but at high speed you may loose some.....
Others likely have different ideas, and that is what makes this a great place. I am personally very happy with my Avon tires.... ooh and when they get heated up.... you can smoke them real nice off the line too!