Dino,
   I have the same low airflow problem with my ES's heating.  One thing that
may be contributing to it is that I have a T fitting in the air hose to my
heat muff that diverts some air to my gascolator shroud.  I just used some
duct tape to cover about half of that opening to see if it helps, but I
suspect that you're correct about the lack of a cabin exhaust being part of
the problem.  Please let us know when you find out if your solution helps.
   Skip Slater