my bottom gear was 30x36 and i used it plenty, on all the steep bits, e.g. the 200m 16% section of devil's staircase was at 76rpm and 330w which translated to 7.8kph, so a bit faster than walking, but not much. the steep part of b-y-g is 2.4km at 15.6%... took it "easy" at 5kph (walking would have been ~3kph)
if it was a simple change i'd go for even lower bottom gear.