Ouch. That implies a pretty big disparity between your tanks and everyone else.
No. The numbers exactly offset the rate of ability mod increases, as I said above. If they didn't, the plate armor would become obsolete at high levels.
Plate goes +8-> +11 (at levels 16-25) -> +14 (at levels 26-30).
Hide goes +3 -> +4 (16-25) -> +5 (26-30).
Level, proficiency, and enhancement bonuses are a wash, so we'll ignore them:
The masterwork bump is +3 for heavy armors, +1 for light. But the first comes after 5 (on average) increases in ability scores, which means that both characters' AC's have gone up by 3.
The second comes after 8 (on average) increases, which means you're still comparing +6 and +6.
Assuming we're comparing like vs. like (hide vs. plate, leather vs. chain) the AC difference would evolve like this over levels (I'm assuming characters get their new magic armors at the midpoint of their level range, on average, so levels 3, 8, 13, 18, 23, and 28):
Light Armor AC - Heavy Armor AC
Code:
Level 18 starting Dex 20 starting Dex
-------------------------------------------------------
1-7 -1 0
8-13 0 +1
14-17 +1 +2
18-20 -1 +0
21-27 +0 +1
28-30 -1 +0