Incoherent font for chapter numbers in ToC

In a recent TeX.SX post, @Peter noticed that only the first digit of the chapter number in the ToC was correctly set to \sffamily while the rest digits were in \rmfamily (when the arsclassica style is used). Here is a MWE to illustrate the problem:

\documentclass{scrbook}
\usepackage[style=arsclassica]{classicthesis}
\begin{document}
    \tableofcontents
    \setcounter{chapter}{110}% changed for illustration
    \chapter{Some chapter}
    \section{Some section}
\end{document}

In my answer, I proposed to redefine \numberline to accommodate classicthesis’s implementation. But a self-contained solution is much preferable.

Due to the way classicthesis implements tracking/letterspacing, care must be taken whenever \spacedallcaps and \spacedlowsmallcaps are used. Instead of changing \numberline from tocloft, the following takes care of the chapter number:

% Changing `classicthesis.sty'
...
\renewcommand{\cftchappresnum}{\spacedlowsmallcaps\bgroup}% <- add `\bgroup'
\renewcommand{\cftchapaftersnum}{\egroup}% <- this line inserted
\renewcommand{\cftchapaftersnumb}{\spacedlowsmallcaps}% <- no change needed
...

Modifications as the above should be made for \cftsecpresnum, \cftsubsecpresnum, \cftsubsubsecpresnum (because of \MakeTextLowercase) and/or \cftfigpresnum, \cfttabpresnum, \cftlistingspresnum (I don’t see why you have two \renewcommand’s for these three).

P.S. I suppose the original design didn’t expect more than nine chapters. May I suggest changing \settowidth{\newchnumberwidth}{.} to \settowidth{\newchnumberwidth}{0}? The box width is increased by one digit width, which seems more natural for two-digit chapter numbers.

P.P.S. I also noticed that tracking was set differently: With pdfTeX and LuaTeX, we have \textls[160]{...} for upper case and \textls[80]{...} for lower case. But with XeTeX, we have LetterSpace=18.0 for upper case and LetterSpace=14.0 for lower case. Are there some conversion factors used, or are the different settings made just by designing intuition?

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