The only apparent negative effect to kde-base/print-manager after I removed system-config-printer-{common,gnome} was an easily ignored error message - no functionality appears to be missing. This error message is much preferred over being forced to install the various GNOME/GTK dependencies required for system-config-printer. Please make a USE flag so one can install KDE without the GNOME/GTK cruft.
This is fixed in 4.14.2 as per https://github.com/gentoo/kde/pull/75.