The Photo Stream is not synced.
New images that you import to Aperture are automatically added to the PhotoStream, if you have automatic upload enabled, but that is it. If you edit an image and create a new version, you have to add the edited version to your stream - simply drag it there.
Photo Stream is not the tool for this (afaik -- I don't use it). Sync a container in Aperture with your iDevice(s) (via iTunes; each iDevice has it's own Photos syncing tab) and put the Images you want to show on the iDevice in that container(s).
I suggest Smart Albums. Either create a Keyword (e.g.: "iDevice!") and assign it to the Images you want to be able to see on your iDevice, or use Smart Settings that give the result you want (e.g.: all 5-star Images modified in the last 6 months). If you've turned on syncing over Wi-Fi (which you should) the syncing will happen automatically when possible.
My iPad is my portfolio. It syncs seamlessly and automatically with my Portfolio Albums in my personal Aperture Library.
I do sync with the smart album function. It is just recently when I looked at my photostream I thought it would be nice if iCloud could wirelessly and automatically update the pictures with the adjustments. Technically it shouldn't be too difficult for apple to add this functionality on top of the existing upload function of iCloud (I guess).
Thanks everyone. I'll stick to the old fashion solution using smart album + iTunes sync then.