Q. What does the game data contain then?
A. DLC, created tracks, photo's, and replay's
Q. Wait, so you're telling me I will lost my DLC cars if I do this?!
A. No, all you have to do is re-download them when you are done.
Q. How do I re-download them?
A. Yes. Sign in to PSN. 'Go to Account Management.' Then 'Transactions Management.' Then 'Download List.' Just search through that list and re-download them. If you did a game share from someone elses account, well you can't get them back, unless you have access to that account again.