Your example isn't conclusive that what you're saying is true. Your example is just showing that the extended module only appears in the instance's singleton class, this has nothing to do with method cache. It's how the method cache is implemented that matters. The new method caching changes can't be seen from user code.

I'm curious as to whether the ruby-core team implemented method caches for every Object in the system (eg: every instance of User would then have its own method cache because they all would have their own singleton classes) or just objects that are instances of the Class class. I seem to doubt that the former happened because it seems like it would be expensive to maintain a cache for every object, that means a normal Rails request that generates 65,000 strings would all have their own method caches. Only instances of the Class class having their own method caches sounds more reasonable from a performance standpoint to me.

For what it's worth, I've been trying to get an answer about this from a ruby-core member, although my attempts have been quite lazy through Twitter.