I have the "Cycra primal stealth" handguards and I have been over a hundred MPH with them and they were totally fine. They definitely don't flap around or even move at all for that matter. I got them to keep the wind off my hands so I could ride in colder temps, along with the ducati heated handgrips.
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you can see in these photos that the cross brace would be no problem. I have the FT bars.
When I was installing them I thought that when it warmed up again I would take them off, but I actually don't even notice them. And they only keep the wind off. Barkbusters will keep your levers from breaking if you drop your bike but they are 3 times the price