while its not fully set in stone...i saw something about package id's being the same. This was posted on the wb community forums
"You could try to launch steam with the -console paramter or just run "steam://open/console" and enter "licenses_for_app 208650" into the command line. It should give you a "License packageID".
Compare the package number with this list:
https://steamdb.info/app/208650/subs/
It might be 68463 or 56900.
If it is 56900 you get exactly the same stuff as if you bought it right now on steam. But I guess it is 68463 which is a special CD-Key only package. As you can see the package includes 5 apps including the Arkham Knight app itself. The other 4 apps are surely DLC apps. Unfortunately the names are hidden but there are definitely some apps included.
However it all depends on your Package ID."
I did it and got the 56900 one, which is listed as being the same as a store bought copy