My PVC versions now like five years ago. None of the single hose mouthpieces would not produce back pressure and restrict breathing. I know Luis informed me that the area was equal to or larger than the USD curved version . I can only say what I experienced. They were not optimal.
I enjoyed using the early straight mouthpiece version Luis gave to me in the Bahamas, it worked, but still not ideal from a tactile standpoint and it was a straight mouthpiece, a no go for me.
I am happy to hear the concepts are progressing even if it takes longer. Better right than not .
James