tl;dr: availability on YouTube seems to not effect rentals and to boost sales, and availability on Winny (Japanese P2P service) decreases rentals but increases sales.
One disclaimer from TorrentFreak:
One point of critique based on the main conclusions of the study, is that the observed relation only appears to be correlational. This may mean that the results could in part be influenced by significant third variables such as promotion and overall popularity. Since the report is only available in Japanese we were unable to confirm whether this was taken into account.