This is my take on the problem. In the original application all is well unless silly hp is applied. If you look at the way the diff is mounted as standard it relies on the cross bar top mounting and what is effectively a tether for the nose of the diff. As mounted in the haynes as soon as you side step the clutch the diff tries to buck instead of pulling on the tether as in the original and its this that is breaking the beam. ( I think). The nearest thing I can think of that would allow the diff to be rubber mounted and effectively tether the diff nose is by using something like this
http://www.torquesolution.com/v/vspf...S-MC-R56-2.jpg and not restrain the diff nose. This is the third failure of the mx5 beam and in a minute someone will get hurt by a driveshaft stabbing them in the back.
Bob