The magic speed is about 50mph.
Heavier downpours need a little more speed to get the water to blow over the cockpit, and that presents the problem that the wipers are almost as effective as the convertible top and the faster you go the less they clear. The wipers also push the water off to the side enough that a tiny little river comes around the corner posts and falls down onto your thigh. At speed, the top of your head will get soaked while the rest of you remains dryer - not completely dry, of course...... That would be silly.
Not that any of this has ever happened to ME!
My last ride through heavy rain was quite a while ago, but it was bad enough that I carry a bottle of Rain-X even today. The stuff works great!