My personal reason for using those OD's over the GV unit is the gear spread and OD ratio. Those transmissions have a lower first gear than a traditional 904/727 (2.74 vs 2.45), and a better OD ratio. The 200r4 had the best OD ratio of .67 I believe (.73 in the 42rh, or .78 in the GV)... So I see that as win/win.
I would also think that the transmissions alone would have less power loss than a GV OD unit on the back of a transmission. But I'm no engineer or mathematician...