Yes. The Sport 1100 1994-1996, used an 18-inch rear. The factory equipment was the Michelin TX15/25 combo. 120/70-17 front and a 160/60-18 shoe-horned onto the narrow rim.
At the time, the 150 and 160 width 18-inch rears were pretty standard on sporty bikes. In the mid-'90s, though, the 17x5 rear became pretty standard with 170/60 and 180/55 sizes being fitted.
The '99-'01 V11 Sports have a 4.5 inch rim and came with a 170, but as we all know, the 160 usually works better on that rim. In '02, the LeMans came with a 5-inch rim and a 180/55-17 rear tire. Again, per usual Guzzi practice, they shoe-horned a one-size-too-big tire on the 5-inch rim. The 170/60-17 rear usually works better on these bikes.
Anyway, the Sport 1100s need the 160/60-18. Contrary to popular belief, there are plenty of choices for that size out there. Eight or ten sport-touring tires and a couple of sport tires. It's really not that big a deal to keep rubber on bikes with the 18-inch rear wheel, and I'm constantly amazed at the threads on different forums complaining about the the tire size.
If you want sporty, the Bridgestone BT-016 is more tire than most street riders will use.
For sport-touring, every big player has a couple choices in the size.
I've never had a problem finding an appropriate 160/60-18 for my 1996 Sport 1100. Right now it's got a set of the (now discontinued) Pirelli Diablo Strada. They've been great.