My experience with GTA V (a retail copy) is that it's also somewhat finicky regarding install and such. There's a decent chance that you'd put all that effort into trying to patch it with legitimate files from steam or some other install trick and it'd come to naught.
Pirated software comes from changing the legitimate software. The implication for you is that Steam seems to download only on a whole-file basis, so any updates or files subject to these changes would have to be re-downloaded anyway, meaning you don't really save any time by taking those pre-existing files.
Really, in sum, I'd just recommend using the default download offering available direct from Steam. It just skips over so many potential problems trying it any...