Black Kat Cowl has max fire weakness?
Health after FoV's charge:
Seraphic Helm + Ancient Plate Mail (fire weakness: high)
Black Kat Cowl + Ancient Plate Mail (fire weakness: medium?)
Fire deals more damage with BKC.
Health after 4 hits from fire:
Seraphic Helm + Ancient Plate Mail (fire weakness: high)
Black Kat Cowl + Silvermail (UV Fire Low) (fire weakness: low?)
Fire deals same damage.

There are more examples of armour stats lying. Snarbolax, for instance, seems to have a tiny amount of negative curse resistance. Duration doesn't increase, the amount of weapons cursed doesn't increase, but damage dealt to the user does.

Zeddy I think curse does less damage to you wearing snarby is because snarby has shadow resis and I think curse does split shadow damage or maybe more normal defense

Curse doesn't do less damage to you while wearing Snarby; it deals more.
I've tried using a variety of other 5* armours, and it didn't matter how much normal, piercing, shadow or elemental defence I did or did not have. Outside of Snarby gear, only curse resist affected the damage.
And, to clarify, you're bringing this up because the statistics bars on Black Kat Cowl show fire having a -2 penalty, not a -4 penalty?
If this is true, then it would be nice to know. I'd like to have cleaner comparisons, and more data. How many times have you run these tests? Do they always produce the same result? (I'm not an expert on fire.) What about Seraphic+Silvermail? Can we get some 5-star armors in there?