Andrew R Stewart 

And the not mentioned pedal pressures affecting the tension on the chain's upper run being the biggie issue for so many riders. Chain side to side flex (not really a measurable aspect) and chain ring tooth profiles (wear and symmetrical vs ramps and pins) are also factors.

Ft ders have improved WRT the cage plate surface contours, inner/outer plate curvatures and heights and overall cage/body stiffness. The Mountech is a rather "basic" design regarding the above aspects. I have test fitted a few ders on the same bike many times to find the best one for that bike's set up. Unfortunately ft shifting is far less black and white then the rear end is. The bottom line is to experiment with all the aspects that you can and try other ders if still not happy.

BTW one can reroute the cable to a below the BB shell/stay path is really wanted. An under BB cable guide is easy to install and chainstay clamp on casing stops are around for the looking. I won't say that the small amount of lowering the der will make a big difference though and that sometimes the up shift can become problematic with the cage too close to the large ring teeth tops. Andy