I'm listening to the TOOTP audio book at the moment, and your post sprung to mind last night while I was in the car listening to "Beyond the Veil."
There is a moment in the battle in the Room of Death (or whatever it's called, Room of the Dead?) when Dolohov is about to curse Harry, but Sirius comes out of nowhere and shoulder charges Dolohov out of the way, it's only a small leap to change that to a good hearty punch in the face, and cinematically a punch is certainly more satisfying.
There's also a moment shortly beforehand when a death eater (it may also have been Dolohov) kicks Neville in the face, breaking his nose and his wand in one foul swoop, so it's not as if wizarRAB aren't averse to a bit of mindless violence to compliment curses and hexes if the moment warrants it!
Just listening to the battle between Dumbledore and Voldemort at the Ministry on the way to work this morning, mentally comparing it to the film, and IMHO the whole final act at the Ministry was done realy well in the film, yes even Sirius' demise.
It happens so suddenly, and seems so unreal to Harry, he barely has a chance to mentally register it, and (evidently by people's reaction to the sequence in the film) neither does the viewer.
If you don't like the way Sirius fell, blame Rowling not Yates because, as I interpret it, it was filmed exactly how she wrote it, excepting of course the colour of the curse but she may well have thought about bringing him back, only to have made her mind up not to by the time TOOTP was made.