About BD-50 splitting file theory: as I wrote, don't know *if* could affect a BD-R, because I'm aware no printed BD-50 shows this behaviour; of course, a burned test BD is welcome.
Grain: today I did some tests for my Arrival project, and discovered that at 20mbps, grain is practically intact... just a thought, if you want to change your mind and *squeeze* it into a BD-25... (-^,)