yeah i tried myself, it works, BUT saying that you cannot cheat with them online due to the nature of setup of online play, let me explain
For Online you 1st have to have 30 cards, these 30 cards can be made up of any packs that you aquire be it the starter pack, booster packs or theme packs. You can mix & match packs and cards but can only have certain cards determined by rarity, race, ect, ect. This means that IF someone has all the cards they cannot use all the best cards they have because the nature f the setup won't allow it, for example it won't allow all ultra rare cards, ect,ect. These cards must be scanned by the eyetoy before online in the deck builder menu, This stops people swapping cards for better cards online that ain't registers for that online pack. Then when playing online when you get to pick your cards after your 1st 5 cards it tells you what card to choose from your 30, if you don't pick that card and try and swap it for something else it won't register and game will not work.
So basically online play with copied cards just means you will face different cards but not meaning stronger opponents.
i've reserved online all the packs as my two boys really love this game,