I didn't weigh in on the 0L prep thing originally, but I'll give some thoughts since my old pal JCoug did.
I was working in a low-intensity job pre-LS with lots of time on my hands, and I was anxious to get a start, so I read through the E&Es for my fall classes. I also worked through GTM, LEEWS and Delaney's examsmanship book. So, a moderate amount of 0L prep.
It worked out for me. But like others have said, you can never tell what's sufficient or necessary without going back and doing it differently. I will say that cries from the opposition about confusing yourself when the prof says something contradicting what you've read seem unfounded to me. This is largely because I didn't remember the details of what I'd read well enough 3 months later when I actually learned it from my prof. But, it was kind of nice to have thought about the concepts before so I wasn't hearing them for the first time. Like JCoug says, almost everyone will know the material eventually. I do know, though, that I started practice testing a lot earlier than everyone else, and I don't know how much of that was because of my comfort with the material.
What was definitely more helpful, like others have said, was working on examsmanship instead of substantive material. But this is a little tricky in that you can't really figure out how to take an exam until you know at least a little law. You can abstractly understand concepts like "argue both sides" and "look for forks," but you don't you can't really appreciate what that means until you have a complex fact pattern in front of you and some law to work with. So, with that in mind, I think there is value to something like the following approach: read through the torts E&E, excluding the last two chapters. Read Delaney's book/GTM/LEEWS. And then do the last two chapters of the Torts E&E, which is a great primer on how to take a torts exam. At that point, you'll know enough of the law (at least as the book presents it) to be able to write out answers to the problems he gives you. Then you'll have an inkling of the task before you.
To me, the benefit of that prep isn't learning the substantive material. It's learning how to learn when you're actually in your classes. This is important, because 90% of class discussion/lecture is pretty much worthless. There's no way the digressions upon digressions could ever be useful on an exam. If you have a basic understanding of how you'll be tested on the material, you can have a better filter. Each class session should really produce only about half a page of notes: rules, tests, minor variations thereof, and one or two policy notes.
Now, that's not to say that even this will put you ahead of your classmates by December. They'll still learn all that stuff, eventually. But they'll have to sort through piles of slop to learn it. And by having the right approach from the start, I think you can outline earlier and take practice exams earlier.
Like I said, who knows how much of what I did was actually helpful. Just my experience, and I ended up doing well.