LotusJas:
I see no reason we can't just download the extra content now to the PS3's hard drive? You don't to have the game to do that - only to use it, which you can do later when the game is cheaper.
The content isn't available until tomorrow... but yes, in theory, that is true.
If the content is downloaded & then requires manual installation then there should be no reason not to download when you can & install once the main game data is resident in your Blu-ray drive (or installed to your hard drive).
However, if the download installs automatically now, then you may find that the extra content is 'over-written' if you need to install the game upon purchase.
That said, you can always return to the PlayStation Store & view your "Download List" & re-download again (without incurring any additional cost if applicable at that time from the main 'Store listing).
(I will be downloading when the new content is available too; prior to owning the physical game title).
BFN,
fp.