Okay, I can now tell you something else that doesn't work. Instead of double-milling I went for a decoction, as a real-world friend suggested I give that a shot to improve efficiency. While it was fun (I'd never done a decoction before), it did not improve efficiency; in fact, I even lost a point (now I'm down to 71% efficiency). Fortunately, I really like hefeweizens, so I'm going to keep brewing this until I figure out how to get it up to my typical 80%. Next time I'm double-milling the wheat on my mill's finest setting, I swear. Stay tuned.