FOM, you have not answered Irlandes original question - if the engine was performing less well at higher OAT then no -one would be surprised, as you say it is thermodynamic boffin stuff.
BUT, the problem is that at ISA+10 in Irlandes original post the helicopter seemed to be performing too well ie it could hover at at a higher DA according to the graph than you would expect from doing the DA=PA+/- 120t calculation.