When he said that a rich tune is cooler than a lean tune, he might be referring to a gasoline engine. With a gasser, stuff starts melting if you run lean. A diesel melts when you run rich.
Everything you said in your post is correct, you definitely are on the right track.
One thing I found interesting, and I'm not sure if it's true or not, but apparently diesel's start to smoke at about 80% stoich. Meaning, puking some black smoke can make more power than just a light grey haze. You'll hit EGT melting territories quickly if you're blowing thick black smoke though!