space between part title and chapter title in toc [Koma]

Update

With the current KOMA-Script Version 3.20 new styles for toc entries can be easily declared. So you can define an own style for unnumbered chapters with more vertical space before.

\documentclass[
final,
paper=a4,
listof=totoc,
bibliography=totoc,
captions=tableheading,
numbers=noenddot,
headings=big,
chapterprefix=false
]
{scrreprt}[2016/05/10]% needs version 3.20 or newer
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{blindtext}
\usepackage{xcolor}

\usepackage[backend=biber]{biblatex} 
\addbibresource{biblatex-examples.bib}% example bibdatei 

\RedeclareSectionCommand[
  tocbeforeskip=1.25em plus 1pt
]{part}
\addtokomafont{partentry}{\color{red!70!black}}

\RedeclareSectionCommand[
  tocbeforeskip=0pt plus .2pt,
]{chapter}
\DeclareTOCStyleEntry[
  beforeskip=.75em plus 1pt,
  level=0,
  indent=0pt,
  numwidth=0pt,
]{chapter}{unnumberedchapter}

\renewcommand\addchaptertocentry[2]{%
  \ifstr{#1}{}
    {\addtocentrydefault{unnumberedchapter}{#1}{#2}}
    {\addtocentrydefault{chapter}{#1}{#2}}
}

%\usepackage{hyperref}
\begin{document}
\tableofcontents
\listoffigures
\listoftables
\cleardoubleoddpage
\part{Stuff}
\blinddocument
\part{More Stuff}
\blinddocument
\blinddocument
\nocite{westfahl:space}% nur für Beispielbibliography
\printbibliography
\addpart{Additional Stuff}
\end{document}

---

Original answer

Here is a suggestion using package tocstyle that is an alpha version but part of the KOMA-Script bundle. It works also with \addpart.

    \documentclass[
    final,
    paper=a4,
    listof=totoc,
    captions=tableheading,
    listof=totoc,
    numbers=noenddot,
    headings=big,
    chapterprefix=false
    ]
    {scrreprt}
    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage{blindtext}
    \usepackage{xcolor}% <- load this package before tocstyle
    \usepackage{tocstyle}
    \newtocstyle
         [KOMAlike]% parent style
         [entryvskip]% exclude the initial vertical skip
         {myown}% name of the new style
         {%
           \settocstylefeature[-1]{entryvskip}{12pt}% set the vertical skip before a part entry
           %\settocstylefeature[0]{entryvskip}{2pt}% set the vertical skip before a chapter entry
         }

    \usetocstyle{myown}
    \usepackage{hyperref}
    \begin{document}
             \tableofcontents
             \listoffigures
             \listoftables
             \cleardoubleoddpage
             \part{Stuff}
             \blinddocument
             \part{More Stuff}
             \blinddocument
             \blinddocument
             \addpart{Additional Stuff}
    \end{document}

Run several times to get

Another possibility is redefining \addparttocentry and \addchaptertocentry but then there could be page breaks at unfavorable points.

    \documentclass[
    final,
    paper=a4,
    listof=totoc,
    captions=tableheading,
    listof=totoc,
    numbers=noenddot,
    headings=big,
    chapterprefix=false
    ]
    {scrreprt}
    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage{blindtext}
    \usepackage{xcolor}
    \usepackage{hyperref}
    \renewcommand\addparttocentry[2]{%
        \addtocontents{toc}{\protect\vspace{-4ex}}%
        \addtocentrydefault{part}{#1}{#2}%
    }
  \renewcommand\addchaptertocentry[2]{%
        \addtocontents{toc}{\protect\vspace{-2ex}}%
        \addtocentrydefault{chapter}{#1}{#2}%
    }   
    \begin{document}
             \tableofcontents
             \listoffigures
             \listoftables
             \cleardoubleoddpage
             \part{Stuff}
             \blinddocument
             \part{More Stuff}
             \blinddocument
             \blinddocument
             \addpart{Additional Stuff}
    \end{document}

Result: