So mythic raid gear would just save you time rather than giving you a permanent upgrade over lower difficulties. The gear rewards from mythic are hardly worth the effort as-is, this would just make it even less rewarding. Having random 1/3 upgrade procs is fine but mythic gear should always be better than normal/heroic. Doesn't even have to be via ilvl, you could have increased effects from mythic set bonuses vs lower difficulties set bonuses.