A bad synchro is not going to 'lock you out'; it'd grind. And it'd grind at ANY rpm, so you can rule that out.
Only way to know your car is 'normal' is for you to drive another M3, or ask a member to drive yours next to you, since that might be a better sell
. If your tranny feels the same, which I suspect will be the case, then you'd know there's no serious problem.
As for the solution, I suspect you might have air in your system, and here's why. For some strange reason, many cars (especially Corvettes) have slave operation affected by high rpm. I have no idea why since I've never seen an exact schematic (and neither on the M3), but it's common. If you have any air in the system, the issue is exacerbated. Just mention this so you know it's another possibility.
Now let's think outloud. Let's say you have a bit of air, and it's bled out. Now instead of being 'locked out', you'd have the infamous grind, which is a very well known problem. And until now I'm thinking it could be the same Corvette slave issue, especially since our engine spins way higher. Owners resolve that issue with the LS7 (Z06) clutch/slave, but I have no idea what's the difference. Or why that phenomenon takes place (anybody?).
I have an idea to try to diagnose the problem. Here's what I suggest you do and report back to us, so we can continue brain storming
1. Since high engine rpm COULD be the issue, try downshifting to 2nd WITHOUT REV MATCHING. In other words, you'd be fully exercising/testing the synchro.
2. You told us rev-matching doesn't help. Please try double-clutching and see if anything changes.
3. Start a thread and ask a member to please drive your car and his with you next to him under exactly the same conditions (maybe downshift from 3rd to 2nd at 7K rpm? Shouldn't be that fast), using different techniques and different shifting speeds (from quick to slow), and see what he and you think afterwards.
I'm pretty sure you won't have an issue doing it slowly, and my guess is mostly due to the engine having slowed down, pointing to the same mystery slave issue than the regular Vettes. Let's find out. Good luck man.