Yes, I also found there’s an optimum airflow – If you look at plastic melodicas, as well as having a very small opening (after the mouthpiece), there’s a further restriction built into the reed chamber cover. It’s to stop too much air overpowering the lower reeds, and to give a reasonably even amount of air across all reeds.
The shape and size of the mouthpiece will certainly have an (enabling or disabling) effect on the players technique, so I’m very interested to see what Lowboy discovers here. For me, the tone changes considerably when I’m in an optimum state of “alert” relaxation, and the mouthpiece can influence this.
Circular breathing – I find it easier to circular breath with a smaller air chamber, so the smaller the hose or mouthpiece, the better for me!