well, out of an autistic sense of duty, I just tested it. Here's some funny data:
With the help of a helpful lich, I exposed myself to meteors and fireballs (you have no idea how hard it is for them to actually hit you with the spells when you want them to do so) while wearing:
a) No fire protection
b) A cloak of the dragon's heart (fire prot 4)
c) The cloak, + a prot2 helmet + prot1 boots
Fireballs are a bit dicey to lab because the lich's accuracy just about doesn't exist, but it looked to me like I was taking ~125 damage with no prot, ~105 with the cloak, ~115 with all the gear. This is interesting data, but unreliable.
So after fireballs I started paying more attention to the meteors, and here are my findings: with no fire prot, I'd take 16 damage on a direct hit. With the cloak of the dragon's heart, it would be 11. With all the other gear on, it was 13!
Which would suggest that stacking is in fact counter-productive rather than helpful. But that's not entirely accurate, cuz armour value also reduces damage taken by spells. Still, I believe it's safe to assume that if the protection levels stacked, their combination would be stronger than the loss in armour values I had from switching endgame plate to sturdy/mighty plate.
For a final test I took off all armour and compared the divine cloak of the dragon's heart (armour 36, 4 fire prot) to a mighty cloak of dragon's scale (armour 32, fire prot 1).
With the divine cloak, a direct meteor strike would hit me for 71 damage. With the mighty, it was 77. So it would seem the protection levels provide next-to-worthless damage reduction, and you're just better off looking at the raw armour value instead.
In fact, given the number differences between the two cloaks and the meteor damage difference, I wouldn't be surprised if "protection level X" just meant "-X damage"... which is hilariously bad. At least for fire prot. Could potentially be much more useful for lightning prot, since lightning spells do damage tics, but there's no way in hell I'm testing that lol
