I have nothing against supercharging anthing, turbo or mechanically.
There is a large aftermarket to get any amount of power you want out of a
lot of engines, including Subaru's.  The cost of putting a turbo Subie in
is an extra 1000-1500 in engine cost, plus a bunch more labor.  We would
love to do them, but we when quote a price nobody buys one.  Same with a
TDI, although a WRX motor install is still a couple grand cheaper than a
TDI.
     Most of the Subaru turbos available are WRX's.  2 liters and not much
low end torque.  The newer 2.5 turbos are extremely rare and expensive on
the used market.
     Most of the van owners that want big power and are willing to pay the
money for it are Syncro Westy/Sycnro owners.  The turbo Subaru motors
don't fit real well with the Syncro gas tank.
     That brings up the 6 cylinders.  The SVX's are plentiful and not that
expensive on the used market.  They are more labor intensive to install,
but the end result is awesome.
     The newer H6 3.0's are just now getting plentiful and cheap enough to
be affordable for Vanagons.  212 hp and good low end torque also.  They
are only slightly larger than the 4 cylinder Subaru's, and the fit should
be a bit easier in the Vanagon.

     John