Stryfox, If you go back to "On the Wings of XFs" #7 post and punch up the AMX files web site, the first thing you will see is a reprint of Motor Trends
debut road test of the 1968 AMX. In it there is an interesting paragraph that specifically addresses the fade issue.
The AMX came with "twin" piston disc brakes, is that what you meant when you said some Mopars had 4 piston?