WIe @saputello schon in einem Kommentar empfohlen hat: Lass das Paket `titlesec` weg und nimm die Einstellungen direkt mit KOMA-Script Mitteln vor. Beachte, dass Du für `\RedeclareSectionCommands` mindestens die derzeit aktuelle KOMA-Script Version 3.15 brauchst.
\documentclass[headings=small]{scrreprt}
\usepackage{setspace}
\onehalfspacing
\setkomafont{disposition}{\normalcolor\normalfont\bfseries}
\setkomafont{chapter}{\large}
\setkomafont{section}{\normalsize}
\renewcommand*\chapterheadstartvskip{}
\renewcommand*\chapterheadendvskip{\vspace*{8pt plus 2pt minus 2pt}}
\RedeclareSectionCommands[
beforeskip=-30pt plus 4pt minus 2pt,
afterskip=8pt plus 2pt minus 2pt
]{section,subsection,subsubsection}
]{chapter,section,subsection,subsubsection}
\RedeclareSectionCommand[beforeskip=0pt]{chapter}
\usepackage{lipsum}% Fülltext
%\usepackage{showframe}% Seitenlayout anzeigen lassen
\begin{document}
\chapter{Chapter One}
\lipsum[1-2]
\chapter{Chapter Two after chapter This is very long long long title super long title}
\section{Consecutive Section right after chapter This is very long long long title super long title}
\subsection{Consecutive SubSection right after section after chapter This is very long long long title super long title}
\lipsum[1-2]
\chapter{Chapter Three}
\section{Consecutive Section right after chapter}
\lipsum[1-2]
\section{Yet another section}
\subsection{Consecutive SubSection right after section}
\lipsum[1-2]
\end{document}
![alt text][1]
![alt text][2]
[1]: http://texwelt.de/wissen/upfiles/tw_abst1.png
[2]: http://texwelt.de/wissen/upfiles/tw_abst2.png
