To be fair, Rory has actually seen the Doctor in action, in Eleventh Hour, and he's experienced first-hand the way both Jeff, Amy and Rory himself reacts to the Doctor firing on all cylinders. Jeff in particular was a good example of the way the Doctor gets people to impress him, but you also had the Doctor getting Amy to trick her way into the hospital using her police costume. It's not like Venice is the first time Rory actually sees this particular trait of the Doctor in action.