I wouldn't say it's impossible. We've had physics patches in all three Prologue updates, so it could happen. The entire Time Trial Demo was a little over 200 MB itself, so it's conceivable that the two new tracks and a few cars with a GT5 physics patch and an online system improvement could be done for one gigabyte or so. Heck, I'd just like to see the new tracks in Prologue.
A stand alone demo would be easier to do, but Prologue would offer a wonderful basis for a patch, with all the variety of cars and tracks it would offer, over a one or two track demo with a handful of cars. I hope they can do it.