I don't know how feasible it is to split them but a lot of the kcm modules for KF5 are part of the kde-plasma/plasma-desktop package. Since they affect all KF5 applications they shouldn't be part of the desktop package (things like icons, styles, colors, and even baloo settings). After being forced to update baloo the indexer is making my system unresponsive and there's no simple way to turn it off. If at least a binary package could be provided it would make the transition much easier for those of us not ready to switch to plasma5.
The Gentoo KDE policy is to stick as close as possible to upstream. Therefore we avoid to split packages downstream when this is not provided by upstream.
