I prefer medical/scientific evidence based upon repeatable experimentation vs unsubstantiated claims, but to each their own.
Yeah... a relaxed diver will have a better SAC which I pointed out in my first post in this thread; "become comfortable in the water".
If anyone can point to a legitimate source with proper experimentation regarding "breathing techniques and divers" I'd love to read it. So far only a video on the buteyko method for which there is no medical or experimental evidence to back up claims has been posted.
A proper test to confirm there might be something to breathing technique requires proper experimentation; controls, mitigation of variables outside the single variable being tested and sufficient sample size.
All divers as they progress from new, to somewhat experienced, to very experienced improve their SAC without studying these breathing techniques; I cut mine in half in 20 dives without thinking about my breathing; you have to account for this in a real test in order to be able to draw any real conclusions.
Or again, to each their own, whatever you think works for you... although when you say something super specific like "30% improvements" might want to have something to back it up if you expect to be taken seriously.