Going by McCoy's trial statements about being desperate to save him for the sake of peace, it is EASY for me to believe McCoy was genuinely panicked ... it seems out of character compared to how he was when treating the horta (and he SURE didn't understand that character's physiology to any detailed degree), but there wasn't quite as much at stake (well, preventing genocide is pretty big too, come to think of it ... )
The series-era McCoy was cooler under pressure (and for me, the most admirable character, because he stood with his friends in really bad circumstances even though he clearly did not have the resources to defend himself as they did -- something that they got seriously right in STAR TREK 5 when he faces down 'God' after seeing the punishments meted out to K & S.)
But the movie era McCoy, while not having an EMPATH moment or anything quite as good as his cell conversation with Spock in BREAD & CIRCUSES, is really deep. You have him talking to unconscious Spock in SFS and all the levels are peeled back there, indicating some serious level of life fatigue.
I wouldn't call McCoy's TUC character assassination ... not like it is for Kirk and (to some degree) Spock. I think Shatner's mannerism (cut by Meyer) during the 'let them die' speech would have helped preserve real Kirk to a degree, but the movie is more of an agenda with the characters bent and twisted a little to fit that agenda.
It works, but I kinda don't like that it works. Sort of how I feel about the Valeris forced meld, in fact.