The benefit relates in having enough rubber to "match" the power of your bike, on the appropriate rim/tire. More rotating weight (ie, bigger tire) is generally bad, but you wouldn't run a 110 rear tire on the bike either. If you look at the HP of race bikes and how it correlates with tire/rim widths, you see some matching up.
SV650s and 250GP bikes run 160 tires (around 70-90hp?)
600/750s and bigger twins run 180 tires (100-130 hp?)
Open bikes run 190 tires (140hp and up?).
Obviously, lots of exceptions (ie, most AMA factory supersport teams run 190 tires on 5.5" rims).
So, unless you have a pretty built 900 engine, my hunch is there isn't any real performance benefit to going to a 5.5/180.
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