With the lastest Stable Update, “Known issues and solutions”, “Switch to the base-devel meta package requires manual intervention” (pacman -S base-devel) reduced orphans from 15 to 7. Which makes sense given a number of the packages were the result of a limited, but needed, AUR packages.
Executing pacman -S -g 'base-devel' | wc -l resulted in a change of 27 to 18