Greetings from China! Looks like Bay12 isn't blocked here after all (to my surprise). A brief note to say that I'm not intervening or trying to overrule Saint, this is a comment with their invitation.
Regardless of my original intent on the nature of the territorial bonuses, Saint is GM now and therefore his decisions supercede earlier ones I may have made. Indeed, altering the nature of the terrain bonuses to essentially raise a maximum 'cap' on material usage is an excellent idea and in the spirit of the game. More importantly, it does not change what has come before; you only created plate once it became possible to do so with excessive metal. If Saint wants the same to apply to ships, that's their call, although my personal feeling is that a jungle restriction on wood should perhaps restrict high quality wood in ships (making powerful ships but less frequently) and making megaships of some sort. As ever, however, Saint has the call here.
Similarly, if Saint feels that they can balance a cavalry matierial restriction on the plains, they can feel free to do so. Probably this just means either nerfing the nature of the bonus or improving the bonus in the other territories. I won't give details away of the other thread, but from my observation they've done an excellent job of applying such a nerfed penalty without getting carried away in either direction (too weak or too strong).