I have a two piece Hein Gericke Taureg suit which let some water in at the elbows coming home over the moors when it was bouncing down.
When I got home I remembered I have a Spidi water proof oversuit from when I had the Ginetta Roadster... it's one hell of a squeeze to get over the suit, but I haven't felt the need to use it yet. It did however keep me warm in the roadster, but that did have a body and an aero deflector screen instead of the windscreen.
I think you need somekind of armour under it, but it should keep the wind out and be reasonably draft free if pulled in tight.
I've just purchased a buffalo mountain shirt which you wear on it's own, not even with a t-shirt. I intend to try it under my Taureg suit, to see if it's better than the standard thermal liner... the buffalo is also pretty well waterproof.