I was paired with a white mage in The Praetorium when I was on Scholar. I tossed out maybe one heal the entire run not counting Eos using Embrace for 18% overheal. The white mage did nothing but heal and had an 89% overheal at the end. I have never seen an overheal number that high before. I doubt they would have changed their playstyle if they could see overheal numbers on the fly, however. What I wouldn't mind is a post-fight summary that showed your own DPS and overheal stats.