Steve, I know that caching usually helps. But from all test I've made with MediaWiki installation on phpMyAdmin wiki, I can see that enabling any caching slows things down quite a lot.
With memcached based caching, each request takes about 15 seconds, if I add XCache backend for variables, it slows down things to 10 seconds and I receive same results with file based caching. So it doesn't seem to be problem with memcached, but rather MediaWiki issue. Unfortunately it's quite a lot of PHP code to investigate and I didn't find anything obvious in low level caching interface.
So say good bye to variable caching, opcode cache is enough for now and MySQL seems to handle the load quite well.