So its almost official with Tom Warren Confirming Three Columns for the start screens of all devices with screens of 4.5" and larger. He earlier leaked that they were testing it for lower resolution devices as well, but now that seems unlikely to release. As far as authenticity goes, regulars on WPC know that Tom Warren has never failed in his leaks and has unfettered access to WP8.1 so we can assume he knows what he's talking about. Hence I've started a UserVoice request that they allow WVGA device users, who are the majority of WP users by far, the option to enable Three columns on the start screen.
Please Vote for it.
Three Columns/Show More Tiles for WVGA Devices
Please Vote for it.
Three Columns/Show More Tiles for WVGA Devices