A large group study of individuals that are taken from two groups.  A group of people that are currently paleo, and have been for 2+ years, and a group of seemingly healthy individuals that eat the standard healthy American diet.  Two control groups that continue to follow their current plan and then two groups that switch and follow the other plan.  I think it would give us a good idea of how much better a paleo lifestyle can be.

My theory is that the typical American diet folks will get better.  The paleo forced to eat the junk would get worse and the control groups would stay the same.

Of course, I don't see many paleos being willing to participate since it would mean a temporary foray into being unhealthy.

yeah there is no way I would go back.
Look at supersize me... that shocked many health professionals.

Then there was the guy who did a fast food only diet and made the 'healthier' choices and was fine.  But, you don't see his experiment on the news.

http://skeptoid.com/episodes/4088

This is interesting, read the 6th paragraph especially it mentions a higher calorie diet and lots of saturated fat.  I would copy and paste the sectin, but the site won't let me without copying a lot of extra information.