The tooltips for the IA debuff are wrong actually. If you look at the script files, in SM/HM EC, the IA debuff reduces armor by 160% (not just 100%).
I actually look to my character information when looking at debuffs like that and calculate based off of the listed armor rating compared to my standard armor rating.
If the armor debuff is 160%, then Firebrand is identical to the Fabricator droid: Shadows would have the most K/E because they would have 75% of their armor, albeit light armor, compared to the 16% of VGs and 0% of Guardians. I find it kind of amusing that Shadows are the tanks that are the least effected by armor debuffs pretty much entirely because they use light armor (one would assume that my silk gown would incinerate *better* than metallo-ceramic composite armor plates, but apparently that's just not true... :P).