LintMan is right I have noticed the same. I don't know if emissive armor is stacking but one thing is for sure: the emissive factor is compared to the damage of the WHOLE salvo! Fire one weapon a time and it can hold if the weapon is small enough. Fire a salvo (and all fighter Groups do this) the whole salvo is compared against the emissive factor, i.e. the armor stands no chance.
Best armor ratio is still good all titanium armor (or organic and crystallide...).