A couple more points to clarify the training effect ( I am not convinced it will work yet but...)
In my college SCUBA class we tried breathing with a 20" snorkle to demonstrate the effect of water pressure this happens to be 50.8 cm. It was impossible to breath with underwater (less than 50cm given some is sticking out of the water) I believe in the original study they worked with the resistance for 10min per day.

Another variable that is not taken into account is the efficiency of a high level swimmer player. Their ability to move through the water with less effort is very obvious over an untrained swimmer/ hockey player. Given that the high level players wether training or playing may not be pushing their breathing muscles to the same degree. If you practice running/swimming a mile every day you get really good at a mile but after a while your body does not continue to adapt and grow because it is not being challenged any more. So it may be that there would be greater improvements in highly trained athletes who have never used a resistance breathing device/ program

I dont Know we can only guess. (And doug might be r... sorry, I can't say it)
I just love the Science
Brian[/i]