I'm not sure about this study, but there are cases where 'illegal' downloads have actually helped the producer of the product have better sales. When Radiohead came out Kid A the reviews considered the album "Just Awful"[1]. And I think that they never had any music videos or singles for the album, but a leak before the official release of the album gave people a chance to give the album a real shot. They liked it and they bought it. Kid A was Radioheads first release that became number one in the US and it also went platinum in the UK.
[1] http://www.guardian.co.uk/friday_review/story/0,,371289,00.h...