As for the snorkel, I don't think you'll see a real benefit. I asked a cardiologist friend about wearing a gas mask and he said that while I helps strengthen your diaphragm (as I recall), it does not simulate less oxygen- it just makes you work harder to get it.
Exactly right, the other claim that the Training Mask people make is that it increases the surface area of the alveoli, allowing you to take in more oxygen with each breath. I read their web page and the paper they cited. The training mask was developed to train firemen who have to breathe through SCBA's which puts a strain on their pulmonary system.
It appears to help with that, but the limiting factor with altitude is not the surface area of your alveoli for oxygen exchange, it's the surface area of red blood cells in your system to hold the oxygen and circulate it to your muscles. The Training Mask does nothing for that.
