I'd prefer to make the stress be exactly based on a craft check, that way player crafters (seeing as we have one) feel as though creating their own armour is viable and strong.
I could potentially change it to X boxes based on the craft check, with light armour getting [1]'s, medium half [2]'s and heavy 2/3rs [2]'s and [3]'s as you suggested.
The idea I had for weapons was that they were fragile, or at least difficult to keep an edge on, much like real life. You go into battle with a keen sword, and it cuts true for the first few cuts, but once you use it as a can-opener to kill someone in full plate, that edge is gone.
At that point it has basically lost it's 'sharpness' bonus, but you can still use it to take advantage of aspects and whatever. However, if you REALLY need to do some damage, you can continue trying to act as though it's sharp, hacking and ramming it into a joint- but at that point it's gonna break under your abuse.
So to prevent swords from being too powerful, that's what I was going for. It's really good for helping you add on a little bit of damage to a difficult opponent, but if you rely on it to hew through mooks, then it'll just chip on their delicate skulls.
I think I'll have to change it to a system similar to the new armor system though, so that the consequence box fills no matter how far over that stress threshold you do, and it's only abuse after that which puts the weapon itself at risk.