So indestructible I'd say would be a trash one removed from the low end. On the high end I'd assume +skills, % based core stat and some other big power ones may be gone. Which would leave flat core stats, GA potential, resistance and armor % and I could see quality still rolling.
Just my thoughts so far with the system . I've only had two times I was able to transfiguration an item more than once and I hit 1% quality both times on both items... yippy 2% haha