I'm 6' and 134 pounds. With me on, the Xciting handles and accelerates well. With my 200 pound friend on the back, (which puts it right at 334 pounds) we still get there but acceleration is nowhere near as good. I wouldn't want to be on an interstate or expressway with both of us for any long trip. I know the Xcitng is promoted, at least partially, as a touring scooter but, in my opinion, it is near the lower limit of power for riding on interstates and such, even one-up. Maybe I'd feel the same about the Silver Wing too.
For me to feel really comfortable on interstates and expressways, I'd want a whole lot more power. And yes, I used to ride a good bit on those kinds of highways. I only really felt confident when I moved to a 750cc.
I've never been the kind to want to really load up a bike with lots of weight. But I don't tour and have no desire to. Everything is relative to what you're used to and what you want to do. In my opinion, for touring or frequent two-up riding, look for the highest horsepower you can get for the money you've got.