I honestly understand you view on having a full face - but sit in your car, put your seatbelt on pulling it really tight to mimic when the tensioner pulls in an accident and lock it on the inertia reel (or strap your harness up tight).
Once in, try your hardest to get your face anywhere near something hard - I bet you can't.
Your only risk I suppose is either flying glass, which will still get through an open visor or anything loose in the car.
All rally drivers and most tin-top racers have an open face, you need it for your peripheral vision and I'm sure you can appreciate the more of this you have on the Ring, the better.
Halfway house is maybe an open face with a chinguard? Intercom could be useful too..