Well, no doubt about it, throwing a tantrum makes folks feel better. Whether someone not named Connors or MacEnroe play better is up for debate.
I'll put another way, if I start winning and the other guy throws a tantrum it's lights out for him. I never think: "oh no, he's gonna start playing better!"
I will say that in doubles (which I play a majority of the time), I will loosen up and display a minor amount of emotion since the Iceman thing can be disconcerting to partners.