It makes no sense for Tobi to claim he is a 'shadow of his former self' if he's Obito - the latter was never anything special to begin with, since his life was tragically ended at an early age. He was simply unachieved potential. It'd make much more sense for it to be someone like the Elder Son or Izuna Uchiha since they had power which could be deemed 'superior' to that which pre-Rinnegan Tobi possessed. I find it highly likely that someone's consciousness is merely using Obito's half Zetsu body as some sort of vessel.