Personally I would prefer if heavy armor protected you from 1 more bullet, and that movement speed and cost is adjusted accordingly. Bringing in AP will force heavy armor to be too good against non-AP, forcing everyone to have AP. If everyone has AP, you can save some credit on not using armor. When people don't use armor, you don't need AP anymore, and suddenly armor is effective again. This becomes a messy merry-go-round that is not fun for anyone.