I ride my 550 on the highway all the time, and it scares the piss out of me.
Of course, this is all my fault. I have drag bars, which are not the best to lean on, and I dropped a tooth on the front sprocket to get more quickness. So at 75 I'm at about 7.5k rpms, which just doesn't seem sustainable, despite my religious maintenance.
With the stock sprocket and taller bars it was much better at speed, but I used to get beat off the line by a homeless guy with a shopping cart—and for my daily San Francisco gauntlet, acceleration is far more important than top speed. I'm hoping, however, to goose the low end with an overbore kit and some cam love. Then I think the bike would be a perfect size for any road: light enough to zip around the city, but a nice size for the highway as well.