I agree full heartedly with all your comments and backed up research supporting the decision not to install a cage in a car wherein the driver will not be wearing a helmet. For the moment let me play the devil's advocate here:
Superformance R cars for example (any of our cars can duplicate), the cage in the cabin is welded to the A-piller and the top tubes are up against the inside of the spider down the centerline of the cabin, rollover bar is in the engine bay against the bulkhead.
1) Tubing only reduces the impact distance 1 1/2". If your unprotected skull is to hit any of the inside cage tubes then lilkely it will hit the same location of a non-cage spider, A-piller or inside roof panel, all metal in a SPF. The distance of travel between ones skull to the centerline of inside roof panel, upper edge across windshield, A-piller, is a distance exceeding 5", 10", 12" respectively (how close is your steering wheel?)
My point being, if you wear a properly secured 5 pt harness the movement is restricted to a minimum even, as properly noted, considering the contortion\flexibility the body goes through during an impact. The closest and most concerning contact point is the cage tubes along the spider roofline, the tube across the top of the windshield and A-pillar are at a further distance and are less likely contact points when properly harnessed. Now, lets say an impact is enough to make contact with cage tubing but not the extra 1 1/2" were the tubing not there, then not wearing a HANS type device or having airbags will likely result in serious neck injuries. Where do draw the line on risk?
2) As someone commented earlier, these cars are sooo low that most large cars and SUV's will likely end up on top of our cars. Imagine a head-on or fontal side swipe whereby the oncoming car will drive right up the front, practically unimpeded. Have you looked at the shape of your front end? Car ramp comes to mind!
My point being, most our cars are fiberglass spiders that will collapse with the least amount of impact coming over the top, whether from the side or the front. Steel spiders similar to SPF may fare better but not by much.
3) "Rollover", IMO, is not really a concern. If a rollover occurs in a street GT40, wow, cage or no cage you have some serious issues. Probability of contact to your skull against something will be the unfortunate and likely outcome due to a crushed spider. Odds of a bad outcome might be reduced with a cage, again not by much.
I could go on but the bottom line is these cars are flat out dangerous as street cars. If they were as popular as motorcycles they would lead the weekly obituaries on Monday morning. The reason I have two GT40's, I can't sell one one and have a clear conscious that the buyer won't get hurt in it. Seriously, I talk with Jesus each and every time I buckle my belts and I drive defensively ALL the time.
I welcome more discussion with interest.
Tim