Not exactly what the OP is asking, but there is some debate among MTB-ers about what ultralow granny gears are good for. While they're great for slow grinding up relatively non-technical steep sections, once you hit tech and need a sharp pedal punch to get over an obstacle the low gear often doesn't work. Paradoxically when the going gets harder a harder gear is required.

So when I swapped from my old 32T for a 28T chainring it made some of the 25-30% (non-technical) sections of trail around here much more doable for me, but it didn't really help my tech climbing at all.