I did some low budget research about aerodynamics, read up on all the
sources I could find, etc.  So I know a little, but I certainly can't spout
data or numbers, just general principals.
 One of those principals, quite general, it that anything that upsets the
air flow around a vehicle and causes turbulence will increase it's
drag...and that anything upsetting the airflow and making it go "turbulent"
causes more drag if the turbulence begins further forward on the moving
vehicle.  Generally speaking.
   So that Westie luggage rack, being right behind the front windshield,
makes the airflow go turbulent right there,almost at the front of the van.
Bad aerodynamics.   Smoothing that sunken roof would certainly have a
significant effect on aerodynamic efficiency for the van, at highway speeds.
  One guiding principal of aerodynamics is the front of the object is more
important, when it comes to minimizing turbulence (read: drag)   That's why
modern cars have flush windshields now, not those sculpted in ones like in
the past.  No rain gutters now..Smooth flowing headlights now..All in an
attempt to smooth the air flow to reduce turbulence, to cut drag.
  If I had a real westie, I would certainly fabricate a flush filler piece
for the luggage rack in the top.  Maybe Plexiglas or plastic of some kind,
attached with some Dzus fasteners so it could easily come off when you
wanted to carry a gas can or a chair up there.  It'd probably be good for a
mile more for every gallon of gas...(total guess there, but with some
experience)
  Don Hanson