Questions:
1) Have you been to Italy before? If you haven't been to Rome before, TWO days there is ridiculous. You could spend a week there and not see everything.
2) What are you interested in seeing? If you like historical sites, art and culture, your itinerary is lacking. One day in Florence is silly. You plan to spend a week in a villa outside of Siena. Love Siena, been there and easily did the city in two days. What are you planning to do for a week? I am not a person that goes to Europe or anywhere to relax, I can do that at home or in the USA for a lot less $$$. If you want to explore Tuscany, then have a plan to do so. Renting a car in Italy can easily result in getting a ticket for a huge amount a year from now, so be sure you know were you can and can't drive and watch your speed.
3) Sorrento, love it, been there, stayed there. It is great for exploring the Naples area. Visit the Amalfi Coast, Capri, Pompeii, etc. Your travel time is not shown on your itinerary. Three nights in the Naples area is not enough.
Again, Rome and Florence, as well as Venice are to me the big three in Italy. Rome has the Vatican, many ancient Roman sites (Coliseum, the Forum, Capitoline Hill Museum, Pantheon, Trajan's Market, many ancient cathedrals and churches, St. Peter's Basilica and the Vatican Museum with the Sistine Chapel. That is just the high spots. By the way, the Sistine Chapel is amazing, nothing like it in the World. Also, climb the dome of St. Peter's. Florence has two amazing art museum with Michelangelo's David in Academia and more art in the Uffuzi. The Duomo (Cathedral) takes hours to explore and there is far more.