Adjusting S column in siunitx

Question

I am a beginner in LaTeX, trying to use it for an industrial test report. I have numerous tables in my document, and I have recently discovered the capabilities of siunitx. I would like to use S column whenever possible. I have tentatively modified one table to experiment, but I find that the options I have choosen make a table with fairly large columns and some no longer fit the page. Also, it does not seem the S columns are well centered in the attached example. What fix can be applied?

\documentclass[a4paper, oneside,11pt, english, numbers=noenddot, captions=tableheading]{scrreprt} 
\usepackage[T1]{fontenc} % Usual fonts
\usepackage{tgheros,textcomp}% Fonts
\usepackage{siunitx}% Handling Si units
\usepackage{tabularx} % More elaborate form of tables
\usepackage{multirow} %handling of fusionned cells in tables
\renewcommand{\familydefault}{\sfdefault}%selecting default font (clone of helvetica)
%-----------------------------------------------------------------
%BEGIN CONFIG 2 CAVITIES--------------
\newcommand{\ConfigTwoGang}[3]{%
AAAAAAA &   1-20        &#1     &1-20       &#1\\
BBBBBBBB    &   1-8     &#2     &1-20       &#1\\
CCCCCCC&    1-20        &#1     &Power      &#3\\
}%
%-------------------------------------------------------
%END CONFIG 2 CAVITIES--------------
%------------------------------------------------------
% BEGIN INSERT  TABLE TWO CAV 
\newcommand{\TableIRTwoGang}[4]{%
\begin{table}[#4]%
\noindent 
\begin{center}
\sisetup{table-number-alignment=center,table-figures-integer = 4}
\caption{Insulation resistance (\si{\mega\ohm}) - 2 gang versions}
\begin{tabular}{|c|cS[table-comparator = true]|cS[table-comparator = true]|}
\hline
\multirow{2}{*}{Part number}&\multicolumn{2}{c|}{{Cavity 1}}&\multicolumn{2}{c|}{{Cavity 2}}\\
%\hline
&Position&{Ins. Res.}&Position&{Ins. Res.}\\
\hline\hline
\ConfigTwoGang{#1}{#2}{#3}%
\hline
\end{tabular}
\end{center}
\end{table}
}%
%------------------------------------------------------
\begin{document}
\TableIRTwoGang{> 1500}{> 2000}{> 5000}{H}
\end{document}

Answer 1

What fix can be applied?

In the MWE you provide, you make only minimal use of the capabilities of the S column type that's provided by the siunitx package. In such a case, it may be best to just use the instruction

\sisetup{table-parse-only}

and omit all further options. The following, slightly modified form of your MWE shows that with this option in place, the contents of the two S columns are nicely centered, which is what I understand you want.

\documentclass[a4paper, oneside, 11pt, english, 
   numbers=noenddot, captions=tableheading]{scrreprt} 
\usepackage[T1]{fontenc}
\usepackage{tgheros,textcomp}% Fonts
\usepackage{siunitx}% Handling Si units
\usepackage{tabularx} % More elaborate form of tables
\usepackage{multirow} %handling of fused cells in tables
\renewcommand{\familydefault}{\sfdefault}%select default font (clone of helvetica)

\newcommand{\ConfigTwoGang}[3]{%
  AAAAAAA  &   1-20    &#1   &1-20    &#1\\
  BBBBBBBB &   1-8     &#2   &1-20    &#1\\
  CCCCCCC  &   1-20    &#1   &Power   &#3\\
}

\newcommand{\TableIRTwoGang}[4]{%
  \begin{table}[#4]%
    \sisetup{table-parse-only}
    \caption{Insulation resistance (\si{\mega\ohm}) --- 2 gang versions}
    \centering
    \begin{tabular}{|c|cS|cS|}
      \hline
      \multirow{2}{*}{Part number}
        & \multicolumn{2}{c|}{Cavity 1}
        & \multicolumn{2}{c|}{Cavity 2}\\
        & Position & {Ins.\ Res.} 
        & Position & {Ins.\ Res.}\\
      \hline\hline
      \ConfigTwoGang{#1}{#2}{#3}%
      \hline
    \end{tabular}
  \end{table}
}

\begin{document}
\TableIRTwoGang{> 1500}{> 2000}{> 5000}{H}
\end{document}

A separate comment: With your current font choice commands, viz.,

\usepackage{tgheros}
\renewcommand{\familydefault}{\sfdefault}

you end up having (a clone of) Helvetica as the text font and Computer Modern Sans as the math font. An inspection of the weights and shapes of the characters in columns 1, 2, and 4 vs. those in columns 3 and 5 reveals some differences; these differences aren't drastic, but they are noticeable. (For instance, the glyph "1" has a "footer" in Computer Modern Sans but has no footer in Helvetica.) Some people may find the use of two different sans-serif fonts within one table a bit distracting. If you need to use sans-serif fonts throughout your document, you may want to employ the arev font package, which sets both text and math fonts in a sans-serif font which, IMHO, is very well suited for the display of numeric material in tables. The image below shows the same table as before, but now with the command \usepackage{arev} used in lieu of \usepackage{tgheros}: