Well, it seems that we got a little off topic here, but hey, so what.
I've been using Ethernet to send music to the player.
I did three trials to benchmark the "uploading tunes" portion of syncronization.

The average of the three was 24.6 Megabytes per minute.
- Call it 25 Megabytes per minute
I know this is not the standard unit for transfer rates,
but I feel it is a more useful benchmark number.
If you have 100 Megabytes of stuff to put on the player,
it'll take 4 minutes (or so)

This benchmark does not include time for
reading / deleteing / rebuilding database, etc,
as those times will vary depending on database size,
ammount of data on the unit, disk size, etc.

I haven't gotten around to benchmarking my USB connection,
but I suspect that they will be significantly slower.