This is just one study so all the usual caveats apply, and it focused on Manga specifically, but the conclusion seems to be that the availability of comics torrents negatively affects regular single issue sales but actually increases trade/collection sales (at least, in this case, for series that have been discontinued; again, the fact that the study focused on Manga introduces variables not necessarily applicable to US superhero comics). This does make sense to me and is actually pretty obvious. The torrents allow people to sample books and decide what they want to own.

(Personally i still need to hold physical copies when sitting down to read, and for my project i prefer single issues to trades, but it's nice to have digital scans available for quick reference and screenshots. But my peculiar needs make me an outlier.)