Yeah, in theory, I agree that team would be really good. FWIW, I look at a variety of metrics that try to explain efficiency margin--Basketball-Reference's win shares (calculated both the way they do it for college and for the pros), a
statistical plus-minus method, and so forth, and
generally but not always they put Harris and Mitchell in the top-15. PER, which doesn't try to account for defense aside from steals, blocks, and rebounds, doesn't (the others adjust the levels of players up based on team defense that isn't accounted for in those stats--like the dRTG Ferry referenced).
But it's for sure an inexact process, and individual stats can probably only explain half, at most, of a player's defensive value. For example, Fair in particular tends to look pretty bad on these sorts of things (not top 15), but I think he's a pretty valuable defender in their zone.