Cable routing, unless done with insufficient radii, is unlikely to be the culprit, as if done wrong it would have been apparent from day 1.
Don't discount a sticking throttle pedal. The non sealed footwells and under dash area of the M3W allow all sorts of dust and debris into the pedal area.
After a newly developed issue this spring with a slightly high (1200-1400rpm) idle, a bit of sleuthing revealed the throttle cable had zero slack at the throttle body (there should be just a small but discernible amount) and traced the issue back to a sticking pedal. A bit of spray oil at the pedal pivot later, the pedal was freed off and normal idle returned.
One of my many winter projects this year will be to remove and clean/plate/repaint the pedal assembly and see if I can't come up with simple mods to facilitate periodic lubrication of the pedal pivots without standing on ones head.
