chemformula, height of plus sign

How do I get the plus sign to be aligned with the arrow i.e. change the height of the plus sign?

\documentclass{scrreprt}

\usepackage[english]{babel}
\usepackage{chemformula}

\begin{document}

\ch[plus-space=8pt]{CH2=CH2 + 3 O2 -> 2 CO2 + 2 H2O}

\end{document}