A related (though perhaps conflicting) observation: I have a Premier and an HD XL, both with Tivo N adaptors.
If I copy a high def program from the Premiere to the XL, and try watching it immediately (while copying), it pauses frequently, and I have to wait while more of the show is transfered. I haven't officially timed it but I'd estimate it takes about 2x the show length to copy it.
But if I copy the same high def program from the XL to the Premiere, and try watching it immediately (while copying), the copy operation is faster than the play time. Not only does it not stall out, but I can even skip some commercials.
So either the XL can put the data on the network faster than the Premiere (contrary to what you observed), or the XL can write the data to the disk faster tha the Premiere (something I wouldn't expect). Or there's something else going on with the network -- though I don't think so.
I haven't tried including Desktop in the mix. I'll do so if I get a chance.