Tobi implied that he knew Kakashi took the Sharingan from Obito, therefore it is 'borrowed' from Obito, and it is not Kakashi's own Sharingan.
The way he says it suggests to me that he's aware that non-Uchiha's cannot use the Sharingan as well as an Uchiha could, and doesn't need to worry about Kakashi using the full extent of the Mangekyo's power. xd