re: 5:15 hour layover in Lisbon - venture out?

That sounds too tight to me. Unless you really know the airport or city well, not worth the risk. I would be too anxious the entire time to enjoy anything. By the time you get off the plane and out of the airport and into an Uber, how much time will you have before you need to return again? And remember you have to go back through security so you probably need to be back a couple of hours before the flight. I just wouldn't do it.