Yes, I am glad this came up because I forgot to ask about it as well.
I had thought that "down side" meant "disadvantage". Of course, my translation doesn't make sense, but I wrote it before I started playing the game. Since so many advancements in OXCE and this mod have taken place in the past year (the last time I really played any X-Com), I thought there was some new mechanic being implemented here that I just haven't seen yet.
Anyway, Nord, it sounds like you are trying to say that the underside of the armor is the only armor value that is considered when this effect is taking place. Is that right? And is the effect supposed to be that aquanauts eventually pass out from high water pressure while wearing the Aqua-lung armor? In the playthrough I just started, I did not experience this effect. I typically had at least one aquanauts wearing the Aqua-lung armor, but no one ever passed out in at least 20 underwater missions. Also, I am not sure that the -20 Stamina effect for the Aqua-lung armor is implemented either. (I really do like this armor, by the way)
Not directly related to this, I have seen the OXCE high stun indicator working recently (the purple number icon), but from stun grenades, etc, and again not from simply wearing the Aqua-lung armor for too long.