It was a plothole in the second movie because the predator did have Thermal Vision without the mask.
And, if it were true that the mask was used for breathing assisstance, why didn't he cough as he laughed? Surely a bullet wound in the lung would limit your ability to talk or laugh without at least coughing, right?
I believe the mask was filled with some kind stimulent. Getting shot 4 times in the chest with a shotgun has to take a lot out of you.
*sigh* If only I was witty enough to think of something to say here.