I contacted Microsoft and from what I can read you are at risk of a ban if you play the game early, even if it is a legitimate copy. Below is the transcript from my chat with customer support.
Theresa: Thank you for contacting Xbox Customer support. My name is Theresa. Please give me a moment to review your concern.
Theresa: Hi Joshua, I hope you're doing good today!
Theresa: I see here that we have inquiry about playing the GTA V prior on the official release, is that right?
Theresa: As per checking, yes it is true that accounts will be banned once they play the game prior on the release date.
Joshua: What is the earliest that you can play?
Joshua: And you will be banned even if it is a legitimate real copy?
Theresa: As soon as Rockstar, game developer release it.
Joshua: So 12am in my Country on the 17th?
Theresa: About the legitimate copy, yes we can still get banned as this violates the rights of the game.
Theresa: In Uk, let me double check that for you Joshua.
Joshua: That seems a little harsh. I know lots of people have played other games early and not been banned. Is this strictly for GTA V? And okay.
Theresa: Thank you for waiting Joshua.
Theresa: The release is indeed on September 17, that's is correct.
Theresa: I understand where you're coming from. I'm also a gamer myself and I feel the excitement to play this expensive game.
Theresa: It's true that not all players are banned for playing this game prior on the release date.
Joshua: Oh okay. If I was to play the game on Monday the 16th would I be banned? And what do you mean by violates our terms of service? Do you mean people leaking videos or people with illegal downloads of the game?
Theresa: In my opinion, 17 is near so might as well play it on the release date to ensure my account can play it the longest time we can play.
Theresa: There's a possibility that it might or might not get banned. But I would suggest that we do it on the right time.
Theresa: Violates our terms use, means there were actions that these players has done for example leaking of videos.
Joshua: So on the off chance that I do get it on the 16th you advise me to not play it until 12am on the 17th or risk a ban? Correct?
Theresa: Yes Joshua, I recommend that you wait until 17.
Theresa: My account was actually suspended before and believe me you don't want that to happen on your account.
Joshua: Okay thanks for all of your help. I think I'll wait. I have been waiting for years I can wait a few more days.
Theresa: Exactly. Good thinking Joshua.
Theresa: Might as well use the time to research the game.
Theresa: I’m glad we were able to resolve your inquiry today.
Theresa: It has been a pleasure chatting with you.
Theresa: Are there any questions that I can answer for the mean time?
Joshua: That's a good idea, thanks. I am glad too. Thanks for the confirmation and likewise it has been a pleasure talking to you.
Joshua: Also I think that is about it. Thanks again and have a nice day.
info: Your chat transcript will be sent to my email at the end of your chat.
Theresa: I would appreciate if you can click on the black X symbol next to the envelope to close this chat session.
Theresa: This is good as resolved.
Theresa: Thank you for being with me all this time.
Theresa: Thank you for contacting Xbox Customer Support! To end your chat session please click the above the chat window. If you have any additional questions after you disconnect, feel free to contact us again. Have a great day!
Joshua: Will do and take care.