Function as well. So, if I recall the flow rate of a Conshelf first stage was around 80 cf (from memory, looking for data, have any please?). And all of these being discussed are really just fancy Conshelfs. The volcano orifice is fairly small and is partially occluded by the pin. The Mk 2 Evo is 106 at 3,000 psi and simpler yet. And a G250 second stage will not notice a swing from 145 to 125 on IP. And the Mk 2 Evo is also small and light-ish.
I have the Core Supreme (ACD) in DIN. I need the yoke kit for it but AL makes conversion difficult compared to Scubapro. It is a very tiny first stage considering it has ACD and is sealed. Pretty cool. Granted a Scubapro Mk 11T is even cooler!