I have a Icon Primer Pack. It also has straps that attach to it, so it can be carried away from the bike (backpack style), when not in use.
Yes, a Magnetic bag will scratch the paint. When you lift the bag off the tank, lift the magnet portions right up, don't let them slide. Today's clearcoat isn't what it was.
I had my tank bag slide -most- of the way off the tank once. Never at Highway speeds. In fact, today, I sustained about 150kph for more than a couple of minutes and there was no sign of shift. (Cops either.
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When I had it shift, I was riding the snottiest twistiest most gravelly potholed piece of crap mountain road (read: nothing at the top to make the park money, so it was about 10 yrs overdue for a rehab. There were even parts of the road that had collasped, and they just put signs and paint around them.), and I found it bounced all the way to one side, and one of the magnets hanging off the side of the tank. I didn't actually lose it, but my tank wrap doesn't quite look the same.
Actually, coincidentally, that road was also the one road that I had found to date where I will -not- ride it again with the 919, but could probably have a ball with it all day on Dita. Of course it would be trumped by a smoother road on either bike. I look at it though that a beautiful day in the mountains beats a day in the city any time.