First of all congratulations on your very dear admit! UNC is a strong program and it is a Top 20 one too!
I know the universe likes round numbers - top 10, top 15, top 20, etc but I don't know where the imaginary line between top 20 and top 15 lies. I don't think there is really a monumental difference between UNC and Cornell or Stern. They are all top 20. I would not worry, doubt or question your success.
What I would question is why do you want a Full Time MBA? Why not Part time or EMBA? Those are easier to get into and they don't put as much emphasis on your GPA. The downside of course is that they don't help you look for a job, but you already have a good one and you are planning to stay within MBB - does your employer have an MBA or Tuition assistance program?
P.S. To be somewhat pessimistic, which I will be, I would say your chances will not improve much next year due to age - you are entering the dangerous 30+ territory. Instead, if you want to give it all and your best, and if you are still doubting UNC, you have a few weeks for the R2 - get a consultant. Pay them a few grand just to say you had one, and give it the best shot you can, and if you can improve your chances, then great. If not, you did all you could, and you never need to doubt your options again - you got into top 20, you did your best effort, and that's the end.