Ok, here's some new input from me. I did a test today with a test cube that always produces gaps. I printed it at 220 C and marked the filament at the beginning and the end of the print. I then did the same thing but let the filament go out into the air (without bowden tube or anything). I also took pictures of the filament pattern on the two runs.
The amount of filament that was pushed through the hotend when printing the cube was 44.4 mm while the amount for the free filament was 51.3 mm! Cura says that it is supposed to be 51 mm. In other words, there seems to be a fairly large amount of slipping during the print.
Here is the picture of the pattern on the "free" filament:
and here is the pattern of the filament that went through the bowden tube and hotend:
I unfortunately reversed the filament to get it out of the machine so there are two sets of pattern. There is still a pretty big difference in the patterns though and you can see that the "diamonds" are skewed.
So, the question is how to solve this. More pressure on the filament? A better knurl pattern?