Impressive! To be fair, though, the metal fan mount that comes with the UM2 isn't very efficient. It has large pockets where air gets trapped and gaps where air escapes...
This is true.
As the fans cant really compress the air they don't like pushing against anything. Having a flat surface so close isn't ideal and most 3d printed shrouds with 45deg fan mount have a flat surface very close.. Also you will notice that the fans are a lot quieter when horizontal as there is no side loading.