I don't think Tobi is saying he 'is' Uchiha Madara.
Tobi: "The Sharingan's true power...I, Uchiha Madara's power."
is not really correct.
このうちはまだらの力が (konouchihamadaranochikaraga) is the last sentence.
この (kono) does not mean "I". It's hard to explain in english, but...