You need to go back an read what I posted; I never mentioned 400ex plates did I...? Only stated an "Old Honda Team Trick". Honda Teams had numerous clutch components (as well as just about every other part), that did not exist (yet or ever) for public consumption, that went for all the factory teams; Kawi, Yamaha, Susuki, ect... Thicker Varying Plates, Thicker Steel or Aluminum Plates, Varying Spring Lengths & Tension, Different Clutch Pressure Plate and Centers, Baskets, Washers, ect...the full range of Clutch components to increase or decrease Clutch engagement..
Honda Team used 1-2 Thicker Steel or Aluminum Plates + Regular Plates, Varying Springs, and Baskets & Centers to achieve the Clutch Feel & Performance for each individual Team Racer. Could very well be the 400ex thicker plates evolved from all this early clutch work by Honda, not certain or really know. What I do know is what I just posted, and what Neil confirmed over time.