If you are set on Via Napoli, a reservation is a good idea. the Wine Cellar serves food and takes walk-ups. Another excellent option is the nearby Trattoria al Forno at the Boardwalk.
I have mostly been disappointed by WDW TS food, but we've had two excellent meals at the Trattoria. The food and service were both excellent. The only dish that was so-so was the chicken parm.
they only offer a limited selection of pizzas, but they have an excellent oven. For FL and WDW, it is excellent pizza, though they are personal size.
One more option worth mentioning- I have not been there yet, but the same company/restaurant group that runs Via Napoli, Space 220, and Tutto Italia just opened a BRAND new restaurant in the nearby Flamingo Crossing area called Simply Capri. The menu and offerings are similar, and they are open until 10:30/11pm.