Clarence, the biggest problem with your binning argument is that it doesn't explain at all why tuners are not able to get the same level of power out of x20i as the x28i ON THE DYNO.
I am sure some level of binning exist in any manufacturing. But, let's think about this hypothetical binning of yours. The ones that pass the test are the ones that can generate ~240HP for a reasonable amount of mileage, say 100K, without blowing up. And the ones that fail are the ones that can't generate the same power at the same boost, so they would detune it and sell it as x20i.
BUT, this does not mean that the failed ones would blow up as soon as you up the boost to 328i level, even if your conjecture was true. It could mean that when the boost gets turned up, the longevity of the engine will fall below the acceptable level. But that alone will most definitely not stop a tuner from bumping up the power and selling the reflash. No aftermarket company is going to do a 100k mile test to see if the tune holds up.
Now, it could be that BMW has put extra guard against high boost in the software to prevent people from pushing these "defective" engines too far. But if that is the case, then the direct reason for the tuners' supposed inability to get the power is NOT the quality of the engine, but the software. It could be that once they crack the software, the owners with the tune may find that their engines break down after 50k miles. But that hasn't happened, apparently because people can't make big power out of it. This very strongly suggests that binning cannot be the only, and even the most relevant, explanation for this problem.