Mike,
I based my theory off of what I learned directly from an SAI representative that I met back in '98 in Auburn at the fall auction. He stated that at high speeds it was noted that the Cobra's were having problems with the front ends getting light and having a tendency of a floating feeling. He stated that what their research showed was that the radiator angle was causing a lifting effect on the front end. His words not mine.
Now as far as the other cars you mentioned, they benefit from far superior aerodynamics of the overall body than does the Cobra. I'm pretty sure the the typical Cobra has a bunch of "Dirty" air underneath the more modern cars dont have.
I by no means am defending my position as I don't know the correct answer as I also am not an aeronautics Engineer, just relaying what I was told by Don R. from SAI.
I can see your point though and believe it also has merit, keep in mind though that problems experienced by a "lifting" effect on the front end are only going to be experienced at extreme high speeds, not at speeds that the average responsible person that owns a Cobra attain. I'm thinking that the lifting effect probably doesn't start overcoming gravity until well into triple digit speeds.
John