I would be interested to see your findings. If you're not using VAD, then we should be sending RTP packets once per fixed interval. The fixed interval is determined by the payload size. So, first, there should be no fluctuation in the data rate, unless there is significant jitter in your network.
further, the 80Kbps is for the IP payload, and does not include any layer 2 overhead.
What is the test you're running?
Regards,
Clay