For my logic, that sounds about right. (Looking at your initial question, I thought your desire was to ultrapeer whilst you were away from computer.)

For other over-heads, in general, large video files tend to cause the greatest overhead of upload/downloads. But I suspect they are mostly handled by the actual up & downloaders. Since there is constant messaging between the clients to request, to send, to receive, to tell it's received, verify the piece & request another piece, etc. There is no doubt other messaging I didn't mention. I'm not sure if all blocks of data are standardised the same size on the Gnutella network. But in general, audio files use smaller blocks of data & less difficulty of verifying, etc. ie: also less overhead in general. File-size plays a major part there.