I had the right bonnet opening at 160 km/h on the autobahn last year. The front lock, which was not perfectly fitting (pre-owner obviously modified the the rest nose), seemed to be over-stressed by the air pressure - luckily, nothing happened, but the front lock needed to be exchanged. This happened after I de-installed the bonnet belt just several weeks before - as slight paintwork damages were visible.
I think when the locks are properly installed / adjusted (I had this done by a Morgan dealer) the bonnets are safe - since, I drove up to 190 km/h without having same problem again. But, if there is too much leeway/movement possible at the locks, the bonnet (which is definitely deformed at higher speeds, as aluminium is soft) might get the necessary millimeters to allow the lock to come off the rest nose - either get it repaired, or use the belt :-) ...
