I have seen maybe 2-3 posters at most take the real thing and score lower than their practice score on NBME 11. You can even go through the experiences thread and you'll notice that literally almost every single time it will underestimate that person's score on the real thing. It could be because of the obvious reason - that people studied after taking it (i.e. before taking the real thing), or just a coincidence. If anything, I found it to be the hardest one of them.