We don't know yet. If there is a difference in the game itself, Larian could seed booby trapped versions themselves. It could be something like a quest or location that can only be accessed if you agree/argue that (in-game) pirates are unethical lowlifes, or something.
Well, for the former, it's usually a bad move for companies to upload bugged versions of the game to torrents by themselves. That's happened quite a few times in the past, and it normally ends up backfiring. Not to mention, if the company uploaded it, it's no longer a pirate copy -- the company that owns it uploaded it for free distribution, which means they're giving you permission to download it.
The latter wouldn't be a "special surprise" for pirate copies since that'd just be an in game storyline that applies to everyone.
