Helmets + Heroic Aura: Rearranging Armor Class

I am using a house rule regarding AC, with the objective of de-emphasizing the need for plate armor and promoting the active use of helmets (they cost 25 gp in the equipment list, but nobody is sure on what they do). Also I get to implement something that I love to represent, which is fighters getting better AC by their own merits. 

Base AC: 10

Armor: Leather +2. Chain +3 or Plate +4

+1 for using a shield

+1 for wearing a helmet

+1 for being a fighter and having reached the Hero level (4) or having reached name level with any other class

This is the classic AC17. After this, only dexterity bonuses or magical gear apply.

The helmet itself carries no downsides but for being an extra item for encumbrance purposes, and a little more expenditure of money. The greatest upside is that, when is situationally appropiate, you know who is wearing a helmet and who isn't.

It bothers me a little that some PCs can buy the best armor available in the game right from the start, or shortly after. In this way, the minimum and maximum armor levels are the same but hopefully it can motivate players to use plate armor in the battlefield, but have an easier time choosing lighter armor in a dungeon for stealth and mobility purposes. It can also give a good reason to go shirtless with a helmet if you want to play by the genre's staples.