I’ll apologize up front for the long winded rambling post you are about to receive. Bear with me. I’m still learning here.
I worked up a spread sheet that starts with fuel volume based on rpm and data I found in a really old forum post on the moose pump. It covered the defueling with rpm so I worked that in. From there calculated horsepower etc based on fueling (which comes out fairly close to what I’d expect). Calculating air required to burn that fuel which is where turbo stuff comes in.
I kinda disagree with you on being too lean. Horsepower is really only a function of fuel volume so long as you are over stoichiometric on a diesel. It seems like most people set AFR at just above Smokey pig because it requires the least effort for reasonable results. That’s all well and good. All the reading I’ve done so far says there’s no such thing as too lean on a diesel which is completely opposite of gasoline or alcohol. Since the air is basically unmetered, idle AFR may be 200:1. The leaner the burn on a diesel the cooler it is running EGT wise mainly because you are diluting the heat. The fuel will produce roughly the same energy (heat) regardless as long as you are above stoichiometric. The more air you can cram in there the better (cooler).
Factory type tunes tend to stay very conservative (in the 20’s). This is for emissions and warranty issues I’m sure. Turning the pumps up pushes you into the 16-18 kind of range( assuming it’s not grossly over fueled) because while there may be some small change in turbo performance at steady state you really are pushing about the same amount of air because the RPM is not really changing and there is a lot more air than fuel in the combined exhaust (conservation of mass says what goes in must come out, roughly).
So really, as long as you have enough air, it seems to come down to driveabilty. I suppose there could be such a thing as too much air in the sense that you have to compress that air and that is cylinder pressure which can cause mechanical issues, but other than that I think it comes down to practicality.