No......just no...terrible theory.
Tobi might as well be the ramen guy.
Look, we know Kagami is similar to Tobi, like some Uchiha, but introducing him as Tobi at this point wouldn't be good. We know barely anything about him.
1. Uchiha
2. Team Tobirama
3. Dissapeared or died during war
He needs more of a story. Why introduce a random Uchiha as one of the main villains...just no. I'n sure Kishimoto will surpise us with Tobi's identity, but I'm 99.9% sure it's not Kagami.
If you think about it actually who else could it be. It has to be someone who doesn't really have a story yet. Kagami has as much of a story as Obito or Izuna. And Tobi has to be a person who we don't know about, because know think about this if Tobi had more of a back story and whoever played Tobi had more of a story, don't you think we woulda figured out who Tobi is? Kishimotto has done a fantastic job keeping it hidden like he has so far. Whoever it is, just watch, it is someone we don't know much about and that's exactly how TObis I debtors has been kaput a secret for so long.