In XeLaTeX I have a \textsuperscript issue with all fonts I've tested, with regard to the dash. As opposed to what happens in LaTeX, in XeLaTeX the dash does not get superscripted.
This is a MWE:
\documentclass{article}
\usepackage{xltxtra}
\defaultfontfeatures{Ligatures=TeX, Scale=MatchLowercase}
\setmainfont[Ligatures=Common,
Numbers=OldStyle,
]{Minion Pro}
\begin{document}
88--100\textsuperscript{88--100}
\end{document}
This is the result:

Any ideas as to what's going on?

xltxtraloadsrealscriptspackage, that is the culprit.realscriptsactivates OpenType features for sub/superscripts, but most fonts have limited glyph coverage in those features and it seems to be the case with the font you are using. – خالد حسني Sep 12 '13 at 05:37