Some updated information was brought to my attention concerning one of the remaining candidates, the Project B5 RS6-X turbo.
I had thought the compressor wheel on this turbo was 45mm at the inducer and 56mm at the exducer. I learned that it is 42.6/56 which is the same as the RS4-X (and FrankenTurbo F21), but the turbine side is the larger RS6 size. What this means is that the Project B5 RS6-X turbo is dimensionally the same as the BW RS6, but at a fraction of the cost. That makes it an interesting option in the mix.
Next: F21 vs K04