The only thing that i can think of Clif is that inside the HF DC ring there is a baffle. This baffle kind of directs air down to put chips in the bag instead of sending them up into the filter bag, in it's stock operation. Mounting the filter blow might decrease some pressure loss from trying to force large amounts of air through a smaller opening.
If you half the area of a circle the velocity increases by 4x. Resistance usually increases by the square of the velocity so for every diameter half resistance increases 16x. 16 times a small number can add up to a big number. All of this is a guess, i work with water systems not air. Air compresses so it reacts differently then water but the above is a reason i could think of.
Awesome looking material for the top Lester. This piece is coming along great. I wonder if the name of that wood is just slang for the area of Brazil of the same name? I see it used as flooring but, they use walnut for flooring too . . . the ninnie's.
The figure is way-cool and your choice in parts for the panel couldn't be better. I am really enjoying watching this one come together.