If piracy were really a big enough problem that it could massacre sales of a game, information about it would be all over the Internet.
Some of it is. If you look around, independent developers will sometimes post actual numbers and they are often quite impressive. For larger companies, you really, really don't want to give your investors the idea that your income will implode after some random number of days. Ever wonder why you cannot really get sales numbers for unsuccessful titles?
Anyway, companies are actively working to mitigate this effect. This is why multiplayer with a centralized authentication scheme is so popular these days.
But again, you'd have to take my word for that, as I naturally can't post anyone's real numbers.
In any case, go to a pub with a bunch of game devs. Get them drunk, then ask.