I agree hirez vs. RBCD differences are small, but not necessarily of the same magnitude in all cases. Here are some possible scenarios for a comparison of hirez vs. RBCD playback:
1. Native hirez recording played in hirez vs. downsampled to RBCD -
This was Waldrep's test. I believe differences will be very small and detection difficult, as the results suggest.
2. Native RBCD recording upsampled to hirez vs. straight RBCD playback -
I expect small to nonexistent differences, likely even smaller than 1, above. This was not Waldrep's test.
3. Native Analog, presumably tape, recording mastered to hirez vs. independently mastered to RBCD -
Again, I expect small to nonexistent differences as limited primarily by the source material. However, I doubt this happens often, if ever. Usually, if the 2 versions are simultaneously released, the conversion/remastering is done in hirez then downsampled to RBCD, as in 1. Or, the hirez and RBCD versions were done independently at different times, and it is likely there may be differences due to conversion/mastering. Again, this was not Waldrep's test.
4. Native hirez recording played as hirez vs. simultaneous Native RBCD recording played as RBCD, both from the same live session -
Again, not Waldrep's test. This test would be difficult to impossible, as there are two simultaneous masters of the original session material with possible differences other than just sampling rate and bit depth. But, it brings in possible additional sonic benefits from a hirez recording production chain vs. an RBCD chain. Mainly, that might be the A-D in hirez vs. RBCD using full range source material, rather than possibly more limited source material, as in 2. and 3., above.
I have never seen a test like this formally published or comparison samples of it. Anybody? My hypothesis is that this, if properly doable, would exhibit a larger difference than any of the above, including 1., though still likely a small difference, but perhaps a more noticeable one.
I also believe some recording engineers likely have done this, at least informally or partially, including Waldrep. Some of those have decided for sonic or marketing reasons to adopt hirez as a primary distribution format in spite of the marketing and distribution difficulties and niche status it imposes.
Other engineers remain loyal to RBCD as a distribution format, but they see technical and sonic advantage to recording in hirez then downsampling to RBCD as their finished product. Sonically, it would appear that that approach closes any noticeably audible performance gap vs. hirez.