As an aside, have you noticed any improvements from training with a mask on? Is it like how runners might train at high altitude, so when you come back to normal levels you are more efficient?
A good while back I looked at restrictors for training and my recollection is they didn't show any benefit in studies. Unclear why. They're a lot tougher than any respirator I've tried so I wouldn't expect the respirator to have any benefit that direction. Altitude tents apparently do work but are more trouble and risk than I'm comfortable with.
The only thing I could say is I have improved some at working out with a mask on, but it's still noticeable by a large margin more difficult to recover my breath/energy than without.
I don't think I've generally seen that transfer over to performance without one though.
