reinforced shields

while you have more than 1 energy shield you take 40% less damage

when you have 0 energy shield you take 30% more damage.

Thats way too overpowered sir. CI builds will enjoy permanent 40% less dmg (with their totally insane ES pool) and even hybrid builds will get more than enough reduction so the downside will rarely (almost never) affect them. Not to mention Aegis Aurora interaction if what you suggest is a node or keystone and not literally shields with that implicit or something.

Could do something like 3-4% with no downside, and have 2-3 nodes in the whole skill tree.

Combine your suggestion with any flat life regen and Zealot's Oath and you will always take 40% less damage. Your suggestion might be more balanced if you changed the thresholds at which it occurred (i.e. 40% less damage taken when above 50% ES, 30% more damage taken when 0 ES).