Its the partial pressure of the oxygen at altitude that brings about an increase in red blood cells. The rate defining step with all this is the gaseous exchange of oxygen from lungs to the blood, this is driven by the partial pressure of the oxygen in the air.

With the gas mask stuff your bringing less air of the same partial pressure into the lungs. The gaseous exchange will be at the same rate. It does make any active harder but the power output is less so i think the physiological adaptations might take a hit as well.

Psychologically it might have some value the way doing anything thats really stinking hard is. Like Barry Gibson stated in FO training with a gumshield in makes more sense.

Would love top see someone throw up in one it would be ace.