I really don't think that there is any difference in actual performance between any balanced 2nd stage.....be it Atomic, Zeagle, Apeks, Scubapro.
What will be noticed is the venturi boost.
Some like smooth and "natural", some like the boosted rush down the windpipe.
I have found that Scubapro's have a pronounced boost at max venturi.....I personally quite like that but I don't think they perform any better because of it.
Actually I do think there is a difference in actual performance between balanced 2nds, even if it is very small.
I'm with Halo that the breathing comfort and smoothness of a well adjusted D-series (metal orifice) over the course of a full inhaling is unique in 2nd stages.
I guess it is the remarkable design which gives a very high lever advantage ( I didn't measure it yet, but it looks like that very clear) and the case geometry fault properties.
My Ds are adjusted to 0,6 inch/o2 which might be almost possible also with Apeks, other SPs or Atomics,but with them it is not very advisable because of their case geometry fault properties.
Concerning the venturi adjustment I like most those who show a rapid response to higher flow in the 2nds but don't show a venturi override until 12,5 Scfm a minute. This is something you can adjust with some patience on the Ds if you have a Flow Bench.
Another difference between the Ds and other designs is that the cracking effort during a Flow Bench test is not exceeding 0,9 - 1,0 inch/O2, which I see very seldom with other designs who go up to at least 1,2inch/O2 before the venturi kicks in, reducing the inhaling resistance.
No problems with Apeks (almost no problems), Zeagle, Atomics and other SPs, they are usually a bit stronger and more reliable when adjusted to their specific minimum cracking effort than the Ds, but in actual inhaling performance I'm sure there there has never been anything better on the market.
It is said that the Pilot of SP might have been even a better performance concerning breathing easiness and smoothness, but I have not been able to adjust it to the level I'm used to breathe with my Ds.
My technical abilities concerning the Pilots are limited, so that might be the reason.
I think the small differences between high performance 2nds are in praxis not easy to feel, but up to now I could everybody who I let try different 2nds, convince that there is a difference.
If the difference is important enough for most divers to go through the hassle of getting spare parts and finding someone who knows how to adjust the Ds properly, I don't think so.
The others who don't mind get rewarded.
