Baffled I was. Found http://www.thesamba.com/vw/forum/viewtopic.php?t=277650&start=0&postdays=0&postorder=asc&highlight="Block Off Plate in Front Heater" survey. Someone suggested the plate is for backpressure to keep the ram air flaps closed sufficiently for defrost when driving at speed. 

I tested it the whole box out of the car using two shop vacs as the ram air supply. I tried it baffle in and baffle out; the block off plate does indeed make the ram air flaps close. No plate, less defrost air volume. Couldn't test for defrost or heater duct air temp differences out of the car. 

One guy on the samba had about a 10-20F rise at 40f outside air temp w/out the baffle, but he wasn't going highway speeds. So I guess we get a smaller amount of hotter air w/out the flap. I suspect here in the PNW where it's wet condensation to be moved that's the biggest issue, more air volume would be better than hotter air.

I reinstalled w/baffle plate. Just having a new Behr heat exchanger and new fan should make a lot more than 10F difference from before!