I'd also suggest a Least, Lesser and Greater Crystal of Armor Adjustment. The lesser would decrease the armor check penalty of an item by 1 and increase the max Dex bonus of an armor by 1, the Lesser would be 2 instead of 1, and the Greater would be 3 instead of 1. Though, I'd personally go for -1 ACP/+1 Dex for Least, +3 ACP/+2 Dex for Lesser, and +5 ACP/+3 Dex for Greater. That way, you could make heavy armor and Dex-based characters an applicable option. I'd also add to the Lesser Crystal of Armor Adjustment that the armor does not restrict your movement (if medium or heavy armor) thus you can move at your base land speed and use skills like Tumble on said armor, and the Greater Crystal of Armor Adjustment would allow you to treat the armor as lighter for purposes of class-related abilities (heavy to medium, medium to light, light remains the same).
And suddenly Mithral Full Plate of Speed is light armor like it really should be. Approve, I do.