I also have been tracing this minor and variable under extrusion for a few months now on my UM2, and have started to rediscover several small fixes. Bowden tube was one...
Switching bowden tube:
I ordered PTFE tubing from several sources (ebay, E3D, amazon), including a new replacement one from the UM2 supplier webshop here in Sweden. I then took a meter long piece of PLA filament (fresh, no grind marks from the feeder wheel) and started to test, running the filament through while keeping the arch in the tubing as would be on the UM2 while in use. The differences in resistance were striking.
Old UM2 tube (just 3mo old): This was my point of reference or baseline. The filament passed through with some resistance, not smooth but "hacky", where it would slide in slight spurts especially towards the ends where the tubes had been inserted into the feeder and hotend.
New UM2 tube: Smoother slide, some resistance that increased the further the filament got into tube (no surprise).
The E3D tube: this was a noticeable reduction in friction. The filament passed through with much less effort. The E3D tubing itself felt different on the outside, almost a little oily. Teflon can feel this way (new cutting boards and such), but in case there was some oil treatment done, I decided to retest the Old UM2 again:
Old UM2 + drop of sewing machine oil on filament:
I applied a single small drop of sewing oil to the tip of the filament and passed it through the old UM2 tube. Not a lot of difference the first pass. But when I ran the filament through a second time, there was a noticeable reduction in resistance. Almost, as easy to pass the filament through as the E3D tubing, but it did not get rid of the "hacky" movement.
I guess I could hookup a gauge and measure the force, but maybe overkill for this... Difficult to quite the inner tinker scientist....