I don't remember anything about a cap but I do remember Fyrflight(spelling prob off), the item dev, saying that only certain slots (3? maybe was 4) would have the effect on them so he could limit it that way.
I'd imagine it is 100%. A paladin can get the highest, about 126% with a few procs, but would a really strange combination of avatar gear, instance gear, and the plate void shard set to get that high.