I would have been surprised if piracy had really impacted box office numbers. People who like movies like to see them on the big screen, with the big sound, not a shaky, ill-framed, hand-filmed pirate bootleg on their computer screen. If you're willing to watch it that way, you probably didn't care enough to have ever bought a ticket anyway.
But a lot of movies are so insanely expensive to make these days that a studio utterly depends on DVD sales to make a profit; the box office often isn't enough to cover production costs even with a successful theatrical run. And post-release piracy, with illegally uploaded copies of the commercially produced DVDs, DOES severely impact the studio, because people will download a pirated copy instead of buying it if it's the same quality.