Yes, that seems to be the case - once opened, the photo seems to stay at full res, assuming that's optimised res and not the full, original res of the photograph. Not sure about space saving as I installed 8.1 immediately before activating iCloud Photo Library and didn't check before/after storage. Optimised quality, if that's what I'm seeing, looks OK.
You can send the same photo full res on your iPad and try zooming in, you will notice
Anyway, my 1444 photos took something like 3-4 hours to fully upload and load the full res on iPhone .. I think yours will be ok and you won't need to open each one to trigger it to load