[JimMorrison]

Hmmm, I'd say just simulate the particularly intensive martial training, by boosting both Att and Def by +1.

Small resource drop (maybe reduction of 3 or 4?) to all infantry would make it easier to field armies of useful size (and greatly help with initial expansion).

Also, while you may come up with convincing arguments why samurai armors should be AS encumbering as metal plate, I'm hard pressed to understand why they should be moreso.