You can't compare airliners with light corporate aircrafts
Why not, you do (B777)!
Sorry, I dont agree. The PW617's used in most of these VLJ's have an Thrust Reserve of sorts - much like most airliners.
Yes, high temperatures can mean longer take-off runs, lower ROC etc or reduced paylaod but those are the same compromises as the airline world.
Obviously C500td has experience with the Mustang in hot and high conditions but if that is where you plan to operate from the owner bought the wrong aircraft for the job.
SFLY, as you said - the brochure may quote ISA figures but I doubt people are foolish enough to believe they would get ISA performance in Vegas on a hot day. All the figures are in the book if one looks so there should be no surprises (including fuel burn and maintenance costs). If you sign the cheque without doing the research you deserve what you get.
That said, if the Mustang is used what it was designed for I'm sure it's a fantastic tool. I would still rather be flying the Mustang at night in IMC over the mountains than in a TBM 850.