When autopilot was announced (launched), Elon answered some questions and one of those was what happens if the car does not have a live Internet connection. He explained that in that case the data is buffered until the connection is reestablished. This way the system can also capture detailed info on roads/areas with poor Internet coverage. So based on this answer, I assume autopilot data is being sent to the mothership continuously.
After I upgraded to V7, I also had the impression that the bandwidth of the connection improved. In areas where I always had problems listening to streaming audio, there are much less interruptions compared to V6.2.