If you make reservations in advance for the Uffizi and Accademia, absolutely. You don't say what time of year you are going, but the thing that is going to trip you up the most is crowds/lines. Reserve in advance and this headache goes away. You might even have time to visit the Duomo or S. Maria Novella (near the train station), both of which are well worthwhile.
Make sure you bring a good map and I would suggest a compass. Florence is small but it is easy to get turned around and you don't have time to spare.
The train back home depends on several factors: how early do you need to be up the following day? Do you need public transportation back to your hotel in Rome or can you walk? Are you just going to do the Uffizi, Accademia, shopping and take off, or are you going to have dinner in Florence?
I believe when we did our daytrip in December, we had the later 8:52pm train back to Rome. The train ended up being almost half an hour late in arriving, so it was quite late by the time we got back to the hotel. Fortunately the metro was still running.
If you would like more details of the daytrip we did, send me a PM.
