I did a ton of 0L prep (GTM, LEEWS, even read some E&Es), and I ended up doing well. I'm tempted to say that correlation doesn't mean causation, and maybe I would have done well, anyway.
But I'm not sure that's true. I say this because of the difference between my grades first semester of 1L and all my grades since (3L now). While my grades since then have been good, I booked all of my classes first semester, and I haven't booked a class since. I go to school with a lot of smart people, and I honestly think it just took them a semester to figure out how to take law school exams. Once they did that, the playing field was a lot more level.
Everyone "knows" that the exam is what matters and that they need to apply law to fact, but there's a difference between knowing that in the abstract and actually practicing that for months and months. Moreover, I'm pretty sure that having the appropriate mindset from the start helps you learn the material better, because you weed out the irrelevant crap (90% of the lecture).
All that said, many people have been very successful without any prep, and many have been median-pwn3d despite doing everything right. I'm just one data point.