Spend a few hours during a Formula 1 test wandering around the track perimeter road watching the cars circulating and you'll regularly spot personnel from Formula 1 teams, and sometimes Pirelli, watching. And there's a very good reason for that.
It's because, for all the data analysis involved in the optimisation and understanding of grand prix cars, there's real value in seeing how the cars respond on track (on top of the other information they can gather). For all the valid caveats about fuel loads, tyre choice and run plans, a car can only do what it can do. When going fast, it does not lie. This is what makes the 'eye test' valuable.
In general terms, what strikes you is just how mature on track these cars now look. Just 10 days ago, when they were taking their first, tentative steps (admittedly many after a shakedown filming day), the characteristics were more clear and distinct.