Why would a short cage mech make any difference?
When in the two biggest rings (34T front, 32T rear) the mech is in the most tension position (almost horizontal) due to shortening the chain by the looping round both big rings plus one pair of links
When in 34T-11T the mech is pointing downward, so still under lots of tension. Surely because of this (A short cage mech being under less tension in 34-11T) a long one is better?
The main reason is that sometimes the other rings (plus front mech, front shifter, cables and a little more chain) go back on so a long cage mech still lets me run a triple setup should I decide it maybe useful
What is a chaincage?