Don't worry about performance, delete from the wiki and use betawiki. http://en.wikipedia.org/wiki/Wikipedia:Don%27t_worry_about_performance
The files get heavily cached. A page-existence check is faster than retrieving tha page, but in this case, all mediawiki messages are cached at memcached. Performance difference will be minimal. I would expect it to be slighty faster with files but the error level can make it behave the other way.