Pete, You have one on both second stages? A couple years ago during a Rescue Class we had a lady with one on her reg and found when reviewing air sharing that some of the students couldn't use it. It caused a couple to gag and one teenager couldn't even seal the mouthpiece to breath off it. One of the other students also had one that everyone could use though not very comfortable. Seems like a crapshoot if the diver was already borderline panicking to make it more difficult/uncomfortable for them. ( sorry this was hard to write just because of how it sounds)
A friend wanted me to try out her BCD at the end of a pool session. She had plenty of air left, so just gave me her whole rig. She has one of those moldable mouthpieces, and it made me repeatedly gag. After trying for about 5 minutes, I had enough. It was just too big. I have an extremely strong gag reflex that is triggered by the dentist. I've found the Aqualung Comfobite doesn't set it off (good thing I have AL regs!). I even got an AL snorkel for the Comfobite mouthpiece.