Is it worth merging into one appendData() call? 

Something along the (untested!) lines of:
~~~
// Configuration data for view
$app->view()->appendData(array(
    'slim_mode' => $config['slim']['mode'],
    'google_analytics_id' => $config['slim']['custom']['googleAnalyticsId'],
));
~~~

I have no idea if appendData() works like that, but it *ought* to :)