i like this idea a lot (based on the images of the helmets you provided)
but i would do 2 things:
1 - keep the changes subtle, nothing overly fantasy....remember this is a medieval game not conan or wow (helmets were good)
2 - i would implement this with AN OPTION to choose which look you want for your items you heirloom.....you could choose any look 'below' the one you have, so if you had a MW you could choose to have it look like a MW, balanced, tempered, or regular. If you had a tempered, you could only choose tempered or regular....if that makes sense.