Correct me if I am wrong, but isn't it illegal for a police officer to exceed the speed limit while off duty in a civilian car. Police cars (even unmarked) are generally required to have five doors. Do you know if it was a 3 or 5 door?
I might be inclined to think he was on his way to start his shift and was late. Judging by the fact that he was flashing you to get out of the way. Also, due to the fact that he did not officially pull you over, it would lead me to believe he was off duty. As a result, I highly doubt he can submit an official report due to the fact that he himself was speeding, and some might argue, driving recklessly.