Very interesting article and does provide good food for thought.
I fall into the camp that aircraft like PA31s, in the Australian summer particularly, should more be considered like a powerful single, rather than a twin, below 'comfort' height (this 'height' depends on ALL of the circumstances).
Hey, forget about about the reduced ROC or power output with a failed turbo, or the like - I've been in PA31s (very well maintained ones too) that I've popped off the end of some runway in the apparent dead centre of the GAFA and have been convinced that neither turbo is working, despite seeing a healthy 40+" of MP! Anybody who's flown these things much, knows that feeling.
I always based my plan, in those conditions, that I would expect to get a couple of hundred fpm ROD rather than alt hold - on the basis of, plan for the worst - hope for the best.
These things, 40 years ago, when all nice shiny and new, were pretty much promised to do a few fpm ROC at 15c, with Bob Hoover behind the wheel.
Consider, the original P charts only show a SE ROC for a PA31 up to a max of 35C at sea level - hotter and/or higher than that - you are the test pilot. 35c in the GAFA in summer, would have you reaching for a jumper.