Exactly how does it 'stack'? Having multiple components of Emissive Armor with the same damage reduction level results in all those amounts of damage reduction being added together? That's crazy if true. Large ships can become invulverable to everything but the largest weapons and still carry a serious damage payload of their own.
I've been using Emissive Armor in my current Balance Mod game (because my main rival is the Temporal-tech using Xiati
and it hasn't seemed to be working that way, but then I haven't been checking to see how often a hit does no damage. It's not easy to do in real-time combat.
Using a percentage damage reduction doesn't really make technical sense. If you throw 10 times the energy at it, it shouldn't get ten times more effective.
One possible fix is making the components larger. Start at 20 kt, but increase it by 5kt per level so that it gets harder to stack even while the ability increases. You'd end up with 70kt armor at level 11.