Steam is really finicky and picky about their files. No amount of begging from Rockstar will change that :( I'd just download the files from Steam because Steam tries to keep them all up-to-date. Worst case is the amount of time waiting for someone to have a legit answer here (aside from this one) would mean you'd probably have part of it downloaded or installed from the disk into the Steam folder to test. Even when you bought retail copies of games from Valve in the past..when you had to register it with Steam, you immediately had to download from Steam. No disks helped. It was kind of silly. Just a plastic trophy to prove ownership rather than just a digital license.
Short-story, best bet is just download from Steam if you're gonna buy it on Steam.