I have had ongoing challenges with under extrusion with my UM2 since I have received it. The extrusion test is great and I am finally able to have a test benchmark to help make tweeks.
It appeared that the extruder gear was crushing the PLA with the knurled nut and after taking boden, extruder, and feeding system apart, it appeared that this causes far to much friction.
I ended up modifying the feeder because it would not get loose enough even with the set screw all the way loose.
I cut the plastic top down 2mm so It would be less pressure against the PLA and not dimple it as much.
This helped my get above 4mm 3s on blue PLA and had reduced the crushing of the blue PLA. The grey and clear PLA is a little thicker and the drag makes it unusable even in just the hot end.
I may take another 1 mm off the feeding plunger to give me some more adjustable room.