Multiple subsection columns in Beamer (warsaw)

Question

I've got quite a few subsections in one section and they stretch the right half of the header. Hoping to split them up into 2+ columns.

Minimal example:

Some research indicates that this will involve \setbeamertemplate{headline} (source) and probably modifying \insertsubsectionnavigation, but I don't know enough LaTeX to do this. I'd appreciate a hand!

Code:

\documentclass{beamer}
\mode<presentation>
{
\usetheme{Warsaw}
\usecolortheme{default}
}

\title{Sample title}
\author{Sample author}

\begin{document}

\section{A}
\subsection{i}
\begin{frame}
\end{frame}
\subsection{ii}
\begin{frame}
\end{frame}
\subsection{iii}
\begin{frame}
\end{frame}
\subsection{iv}
\begin{frame}
\end{frame}
\subsection{v}
\begin{frame}
\end{frame}
\subsection{vi}
\begin{frame}
\end{frame}
\subsection{vii}
\begin{frame}
\end{frame}
\subsection{viii}
\begin{frame}
\end{frame}

\end{document}