Obito would be impressed with his former sensei's reaction time for a few reasons..
1) why would minato stop growing and improving in skill for 2-3 years?
2) rin was amazed at body flicker in kakashi gaiden, obito"s last days. If any of his pupils were surprised then then they would be surprised two years later after absence. (I admit this us a weak point and ron's exclamation was not necessarily out if surprise but instead awe)
3) still room and reason to believe tobi might be a compound personality so which personality traits or fragments of identity were present during tobi's fight with minato? (We have precident with zetsu, each half is capable of separate and self contained knowledge unshared with the other)
There are ways for it to work to cast aside any doubts of its likelihood.