if I recall factory standard is 20#
the improvement in trigger pull is probably less to do with spring weight and more just the switch from an OEM spring to wolff spring. the reason I say this is I also ran the lighter weight springs then tried out the wolff factory standard weights and found them to be just as smooth. keep in mind the 3rd gen design is sort of like a 1911 in that the hammer spring helps keep the slide forward during firing. dropping the hammer spring will result in higher slide velocity and quicker breech opening, both of which can be problematic.
some of the biggest trigger improvements i've noticed are with HK's where switching from a stock hammer spring to the same weight in wolff makes an incredible difference. it removes all the stacking and provides a much more consistent pull. for me it's not the weight of a trigger that i find annoying, it's when the trigger goes from 4 -> 14#'s in the process of pulling it (i.e. stacking).