I have also seen exhaust hose used for stuffing boxes and so far have not seen a failure.   Perhaps the exhaust hose is being used by yards since they tend to have that hose on hand whereas the proper hose is becoming a bit of a specialty item?  Like yourself I feel that this is a critical hole in the boat so I was glad to spend a few extra dollars to get the much heavier Algonquin hose.  You could perhaps write to Shields and ask about the suitability of using the exhaust hose in this application.  I can tell you that the old hose that I removed on my Maramu was also very heavy wall.

Best of luck,

James

SV Sueno

Maramu #220

Preveza, Greece