"Upgrade" may be the wrong term. Fatter rear tires offer no benefit, and potentially can adversely affect handling (slower turn in, sluggish turn in). I've pointed this out before: watch the moto GP races and note the tiny tires on the smaller bikes. If fatter tires were any better than skinny tires you can bet the GP guys would be running them. Bottomless pockets, endless testing, they run the narrowest tires that can stand the horsepower.
Most SV superbike race bikes that I've seen run a 180 rear tire. The availability of race rubber in that size is a major factor. In reality, I don't know a single person who has found a 180 rear tire to cause problems with turn-in or sluggish handling, it's all theoretical by people who haven't tried it. On a street bike, it's mostly a cosmetic upgrade but won't "hurt" anything as long as you get a wide enough rim to do it, don't try to squeeze a 180 onto the stock rim.
My bike handles if anything more twitchy then it was stock. Better profile out back and lowered front to compensate for a 70 series front tire. I got no complaints. If a bike handles funny it is not setup correct IMO.
Look at the HP of superbikes and look at comparable HP in street bikes and you will see 180s. You can change the setup of the bike to make turn in just as fast with 180s as with 160s. You stand a chance of compromising straight-line stability.
As I said, if you look at the smaller bikes in moto GP (in the 60 HP range, say the 250s) they are not running fat tires. At that level of competition everyone is running what works best. Look at the rubber bands they run on 50s.
I run both on my sv,166-180,depending on what I find in take off pile
initial turn in with 180 is slower.once leaned it makes no difference.
there is absolutly no need for bigger tire than 160 on sv(unless you racing sv monster and are looking for different compounds).