I'm happy I am not alone with this.
At the time I posted the message, I decided to add one quarter tank of 87 octane to my existing one quarter tank of 89 to see if there would be any further difference. So far there is none, but I will certainly stick with all-89 since I am most comfortable with it.
My plugs are very clean, but I do leak some oil through the back of the head gasket, but I'd rather leak oil than burn it.
Now if I could just stumble across an equally simple solution to my awful brake dust problem... (repainting my wheels for the second time this year).