It's lunchtime so y'all get  a post.

I just returned from a 70 mile excursion.  Here's the scoop:

Ambient temp   -1 F (all F temps)
Heater output temp on each speed
1=134 degrees
2=114   "
3=98     "

Rear heater temps unknow as my arm wouldn't reach back there :)

Interior temp = 89

Output temps drop radically at idle

Now, a couple comments.  I have a piece of cardboard in front of the 
radiator for previously stated reasons (I just FEEL better with it 
there) but -and this is key- I also cover the fresh air intake grill. 
 I've found that with it uncovered, too much air comes in at highway 
speed and even with the fan on '1' it blows cool.  This can be 
counteracted by moving the DEFROSTER (top) and FOOT (third) levers 
only partially open but then the wind noise gets real high so I just 
use cardboard in front of the intake.

Since the rear heater uses interior air, I don't think it would 
matter as much what fan speed is used.  A lower output temp will 
occur at the higher fan speeds but that means its extracting more 
heat from the core and adding to the overall heat/mass of the 
interior.  If outside air was being used, the effect would be 
'dilution' instead of 'addition'.

Needless to say, VW didn't over-engineer there heater core 
size/flow-rates.  One point where US made behemoths excel.

-dan   '87 Westy