Hi Dave,
Well, Yes, I do believe long distance mono fin swimming will improve performance. Just think, most dynamic apnea fin swimmers will only ever fin a few hundred metres in a dynamic training session. And maybe only do that once a week. Imagine an olympic swimmer relying on such short distances. Now, imagine your average freediver, putting in say 10 miles in a week, for 10 weeks. That freediver will have developed a feel for the water and an economic style forced upon him as he pushes his blunt body through the water. He will streamline himself. He will also gain strength and fitness, in so much that a 100 meter fin swim will not cause him to puff out. I believe a lot of freedivers simply get puffed out and so samba, simply because they are not trained enough to do the distance regardless of whether they breath and not.
To prove it, I challenge everyone who reads this, to swim normal frontcrawl with breathing, no fins, for a constant 5 minutes (twice the time of a world class long dynamic apnea). Because, most of you will not be swimmers, I bet your arms will be like lead and you will be puffing at the end and you will only get close to the dynamic apnea record, despite swimming for twice the duration). I bet if you tried it breathold, you wont get much past 50m. Its because you are not swimmers. Do you think therefore, that because you wear a mono fin, and have used it for say 3 years, for 20 sessions a year.....mostly doing constant weight freediving in the summer, that you are any good with your mono fin as far as athletic performance is concerned? I reckon you will have not got close to totalling even 10 miles mono finning in your life. So, the solution is simply to put in the miles. Try 100 miles finning in 10 weeks as well as your static (and other training peculiar to freediving as Dave suggests), but ignore putting in the miles at your peril.
Next time your buddies spend two hours freediving the line, why dont you try two hours finning up and down the coastline. You will have finned 4 miles, compared to your buddies who will have finned about 10 x 25/50 meters.
The analogy is a bit like freefall skydiving. The parachutist has made 50 jumps but the time spent in freefall ranges from 3 to 10 seconds each dive. He has therefore accumulated a couple minutes in freefall. Does 2 minutes make him any good?
Oh! Dave, sorry to take this view in the light of your experience (especially compared to mine). But why dont you try a gentle couple hours (not talking endurance, but gentle long swims) see how it feels. Do it a few times and then let us know if it has value at top level.
As for me, I only did 3 maximum dynamic apneas, the last was 8 years ago, 156m. My best static didnt make 4:30. I must have been doing something right to get the distance with such mediocre static ability. The thing is, it is only when you try something different, will you ever get a different result. So dont keep copying what every one else does, because you will only duplicate their results.
I believe long fin swims will add as much value to freediving as the static tables did many years ago, or empty lung statics did not so long ago. So, try something different and see how it benefits you, whilst everyone keeps on doing the same old things.