Top of windows too tall and apparently non-adjustable in kde 5.

[René J.V. Bertin]

I'd also suggest to experiment with other window managers than KWin. Currently I'm mostly using xfm4 with the Mint-X theme, which may not be Qt-based but which fits in very well with my colour palette and font scheme. It also uses FontConfig for font rendering, which means crisp fonts (with Infinality installed), something that cannot be said for most KWin5 window decoration themes (QML-based).

On top of all that it's much less RAM hungry as far as I can tell, or releases resources back to the system much better.

R.