I’m not sure how to respond to this question, but since nobody else has….
There is no doubt that there is significant variation in the loudness of melodicas. A Clavietta, for example, is at the low end, and a vintage Hohner Piano 36 is near the high end. A current Suzuki M-37C is louder than a Yamaha P-37D. Volume is easily measured, of course, and I suppose someone could make side-by-side comparisons though I don’t know of any effort to do this. Perceptions of loudness are not always confirmed by actual measurements, due to the influence of factors other than volume. In my experience, it’s easier to get a bad, honky sound out of a louder melodica than it is out of a more subdued melodica. I gave my Performer 37 away and never played it much, but to the best of my recollection it was not an especially loud melodica in comparison with other brands, maybe somewhat louder than a Yamaha but not as loud as a Suzuki. Unless you really need to project the sound, as on stage without amplification, louder is not necessarily better. Larger melodicas tend to be somewhat louder than smaller ones.