I did a variation of this in September last year. I did 8 days total going from Milton to the Soo, looped superior and then followed the mitt side of Lake Huron to Sarnia and home again. 4000ish km IIRC.
The top end of Superior is beautiful, but it's also cold. If you're going early or late in the year, expect it to be as much as ten degrees cooler pretty much from Sudbury to Thunder Bay.
Highway 17 has a heavy police presence; for the most part you can move at a good clip, but slow down as you're getting close to towns and keep a sharp eye for patrol cars.
Don't ride after dark. The entire route is littered with deer carcasses, and Moose are a reality you don't want to mess with. Sunrise and sunset are when deer are most active, but as long as you can see you have a modicum of preparation and therefore safety. It's the ones you don't see that ruin your day.
The strait between Lake Superior and Lake Michigan was windy as hell. I was on highway 2 on the shore of Michigan so it might be different if you stay more inland.
Gas is a bit sparse on highway 17. I never had an issue with a 22L/400km range, but I recall a stretch of about 130km with no service.
It's a great, scenic route. You wont be dragging your knees at any point but it's absolutely beautiful. Have fun!