Totally legal if done right. Go too fast, and you can be cited for reckless driving or unsafe lane change. People will move over to let you pass. Although some assholes will try to pinch you, which is why it's best to be alert and not go so fast. I was pleasantly surprised at how well the F6B splits lanes, it doesn't feel nearly as wide as I thought it would. I think I was a lot closer to getting pinched on the BMW R1200RT and may have tapped a rear view window or two.

Really helps not to be sucking exhaust fumes or worrying about the inattentive driver behind you. Too many squids do it too fast though, for sure. I've had to move into the lane to let them pass. Their funeral, though.