It seems that if you send a new version of the same .rba project to the 360 after it's already been loaded once, it finishes almost instantaneously, only showing perhaps the last 98-99% of the progress meter before completion.
At first I was concerned that the file transfer was going wonky, but then I speculated that if the audio data remains unchanged, then only the chart/animation MIDI data is sent!
Is this what's happening behind the scenes? Certainly would be the smart thing to do!
Maybe something to note in a FAQ somewhere...