I'd suggest:
5% immunity ---> DR 3/-
10% immunity ---> DR 5/-
I realize that the % stacks, and you could in theory reach 90% immunity to a single element if you really tried to, but in my opinion DR would make the items much more useful in more situations, easier to mix and match multiple resistances and also make finding RIG items more exciting (A helm of +3 AC, Fire resistance 5 is a lot more useful than 10% fire resistance in most scenarios)
It would also be good for PvP balance with high level Evokers, because the resistance couldn't stack to infinity.
Anyway, in short I think in this particular case we could make the game more true to PnP and more fun at the same time
