Dismounted English Knights are already very well armoured, and I think the next armour upgrade is really only +1. The most crucial issue is with those miltia upgrading from nothing to half plate, and the early units upgrading from nothing to padded, which supposedly gives +4. I merely highlighted the fact that CA has coded for the ability for different increases in armour values for different values of armour, wherether it works or not, we probably would not know unless we get some official word about this matter.

By the way, for Dopp, the value for Druzhina and Kataphracts is stated.
Druzhina - 5, 7, 0, 0, 6, 4, 4
Kataphract - 8, 9, 0, 0, 5, 3, 3
First value is base, second is first upgrade, third and fourth is the second and third upgrade respectively, which in both cases are unupgradeable. Fifth value is the defense value, and the last two are shield values, the sixth being melee shield value, and seventh being shield value against missile fire.