Seriously, Vaders breathing didn’t get explained in movies till 20 years after there was 3 movies of him breathing. So what’s the big deal that Grevious coughs? Anyway, that was explained in the first clone wars cartoon. I checked out the dvd and saw it myself. Mace windu crushes his chest with the force.
Two villains with the same kind of breathing problems is simply unoriginal.
I’m sure that George “Poetry” Lucas did it on purpose.