I'm having problems with understanding this. Is that supposed to read "So its natural armor +size modshould equal 11"? And if this is correct, you're saying that you're subtracting the -1 size mod (the same as adding 1). Or else I'm totally confused.

For the armor-temp-HP thing, it's interesting to note that there are a LOT of ways this can be accomplished. Large per-encounter pools, or even a large pool that only refreshes when you take the time to repair your armor, using specific rules. Example: You may make a Craft (armorsmithing) check to repair straps, knock out dents, etc, to restore (craft result) of your armor's HP pool. This takes 10 minutes. That would let you make 6 attempts while your wizard memorizes spells in the morning. I would eschew gold costs, though.

The per-round formula could be worked a lot of different ways, too. So definitely post back if you ever get to test these changes. One thing which smooths out the progression instead of sudden leaps in efficacy at certain BAB milestones is having a formula which increases by level - whether through BAB yet again (not fond of this when adding BAB to AC already), simply adding the player's HD, whatever - and multiplying the result based on armor grade. e.g. double for medium and triple for heavy.