Not going to quote everyone here, but I have a few responses to some of the above posts.
First, don't compare a bias ply tire to a radial. A radial tire is just going to handle better because of how it is constructed.
It really isn't a positive to have a Z rated tire. Z rating means the tire manufacturer has just tested the tire up to a certain speed (depending on where the Z is placed and if there is a Y or W at the end, it varies between 149 mph and 186 mph) and that the tire can withstand that speed for, at least, 10 minutes. Z rating also means the tire will have a stiffer construction, I don't mean rubber, but rather, the inner structure of the tire.
Having a Z rated tire on a R3 is like putting 275/35 tires on a Prius.
A 10mm change in the rear ride height can have a huge difference in handling of a bike. You change the geometry of the bike by raising or lowering either end of the bike. It was common practice to raise the front forks of an SV 5mm to 10mm in the triple clamps to get a better geometry for turn in. Rear shock height was then 13.75" - 14". Sure, you could go with something else, but those measurements really worked for most people. Not just on the track either.
The R3 doesn't have a 4.5" rim. It is 4". Putting a 160 on the rear will really pinch the tire and square it off. If you need to picture this, make a half moon shape with your hands by touching your fingertips together. Then slowly move your wrists together and watch what happens to the curve of your fingers. That is what is happening to a larger tire when you put it on a smaller rim.
As for the KTM having a 150 rear so it must be better because that bike was designed for the track holds no water. The KTM has a different geometry than the R3. That tire may just work better for that particular geometry set up. It doesn't mean it will also work for the R3. Plus, if you have ever visited the RC390 forums, you will see some of the racers switching to the 140 for better turn in on the corners.
If you really need proof that bigger isn't always better, just look at the guys running Moto 3 in Moto GP. Many times those guys are carrying more corner speed than the guys on the big 1000cc 200+ hp bikes. Yet the Moto 3 tires are only 120 on the rear and 90 on the front! Remember, Moto 3 bikes have more HP than the R3, yet those guys are almost dragging their elbows during races.
I guess if you want to throw your money away on bigger tires go for it. I will stick with the 110/140.
Sorry for the rant. >