When talking about Will's potential improvement and so on it's very easy to forget that Will, Eric, Dave etc have actually put in the hard work over the years to work on the flexibility needed (diaphragm, chest, lungs etc) - either through lots of diving or stretching or both. I know from other discussions with others that have participated in this thread that they have developed their own systems of stretching etc to achieve the same. I really don't want to speak on Marcus' behalf but I am sure he is a big believer in this.
It's a fact that for most people here it will be more difficult at a certain depth (depends on the individual) to extract air from the lungs to EQ with rather than using stored air from the mouth. It is also a fact that certain people have dived to incredible depths by EQ by extracting air from the lungs.
I consider EQ as a very individual thing - it depends on each person's targets when it comes to diving and surely their physiology. It's also a fact that for most people it will take much longer to achieve the flexibility needed than to learn to mouthfil.
However, purely in my opinion, if someone starting to freedive is stuck early on at a certain relatively shallow depth with EQ and the issue is due to lack of this flexibility, then teaching them to mouthfil instead of getting them to work on that flexibility is not the best approach and indeed a quick fix. Also (and would be interested to hear Eric's view on this too) it sounds to me that taking a huge mouthfil at a decent depth by the cannonball manoeuvre will not come without risks for the average freediver that hasn't got the necessary flexibility.
The mouthfil has taken many to great depths so I don't think anyone disputes that it works. However, it's a very simple fact that once you've mastered it there is not much more gains you can get from it other than be able to take the same huge mouthfil at a deeper depth - for that one would need to put in the hard work to work on flexibility and so on which, as demonstrated by some, can achieve incredible results too.
For each individual wanting to use mouthfil at any given point in his diving career there will be an optimum depth to take the mouthfil at (the equation is not to hard if you think about it) - there is no reason not to try and extend this optimum depth by continuing to work on one's flexibility. Of course, looking at the depths achieved with a mouthfil (eg Eric's 100+ with air left) it's logical to conclude that for the average freediver, if he can take the mouthfil as deep and as efficiently as Eric (or even 10-15m earlier than Eric) EQ will not be their limiting factor ever!
From the discussion above, taking this mouthfil does sound like very effort-intensive and personally, being a relative beginner, I would be a bit worried doing it. If someone can come up with a method that can achieve similar results without needing this effort or breaking the streamline for me it would be worth learning.
Worth pointing out that the above are purely my own personal thoughts (wouldn't go as far as calling it an opinion because I still have a lot to learn and experience as a freediver)
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