If putting big numbers on the graph is the issue, then it seems its a toss up between twins and a big single. We've seen data for both. If its pulling a living quarters horse trailer loaded with four horses over Wolf Creek Pass, then it seems that twins is the way to go . There are options: downshifting to a gear that keeps EGTs in the safe zone and/or several gallons of water meth.
Why haven't I put twins on my trucks? I infrequently pull anything over the passes around me. Most of my hauling is on flat and level, and when I do need to haul over the passes I am satisfied with the cheaper options. Now if my business changed to where I needed to pull the passes frequently, then I'd definitely install twins. The Artic Air system would be high on the list for my 3rd gen.
It all comes down to the primary use one has for the truck and for the money necessary to complete the upgrade. In my case, right now $2900 worth of cows will produce $1800 of gross income per year. Twins would not add to my bottom line.
Just my 2/100 of a dollar.