Just to clarify: If you're playing Original Sin from the kickstarter early access key; it will be revoked (and thus the game removed from your account) on release. That would allow you to re-enter a CE-version key.
Of course, this isn't the same if you actually *bought* early access on Steam, since then you also bought the game itself.
Most installers of Steam games I know install the game from the disk (saving you the download) though you're perfectly able to just enter the key and download it if you so desire (but why would you in most cases?). Afterwards it would of course download patches. And depending wheter or not the CE DLC is on the disk, DLC.
But the majority of data will come of the disk, lowering your download requirement. Useful for people with slow connections or data limits...
