I took a tumble the last race of the year at Montpelier. I was in a seat with the old-style Kirkey side supports on both sides, which deflect about the same as a net; I know this because I tested the right side with a pretty good shot and I could feel it flex. I could also feel a pretty intense pain above my right eye, which diminished as my helmet rebounded off of the head rest. I was conscious and lucid through the whole episode and when everything stopped I got out and they immediately loaded me into the IU Health bus and gave me a thorough inquisition and inspection, which I passed. Aside from a slight residual headache I felt fine. they told me (and my wife) to watch for any changes and come back if things got worse and they would transport me, but I felt OK the rest of the night. I didn't start exhibiting overt concussion symptoms until the next day but I feel that I was adequately assessed in the period immediately following my accident.
I don't think there's really any way to know how I would have fared either with nets or a full-containment "cell"; I did find it interesting that the SFI spec only covers the deflection of the shell structure without taking into account the padding, but maybe that's because there's already a spec in place for the padding and they're only concentrating on the shell with the understanding that there are other subsystems that must work in concert with that basic specification; the shell, the seat padding as well as the helmet itself, even the head and neck restraint system being used.
All that being said, it seems to me that racing organizations, manufacturers and testing foundations alike are doing the best they can to continually find ways to make our sport safer; while things may not be perfect they continue to improve. If nothing was done until the "ultimate solution" is found the insurance companies may be the ones who dictate whether our not the sport is even allowed to continue...