As an alternative to putting huge rewrite map into applicationHost.config you can put it into a separate config file and then reference that config file from your web.config:
For example, create a file named rewritemaps.config and put this inside of it:
<rewriteMaps>
<rewriteMap name="TestMap" >
<add key="foo" value="bar" />
<add key="goo" value="dada" />
</rewriteMap>
</rewriteMaps>
Then in your web.config reference this file:
<rewriteMaps configSource="rewritemaps.config" />