1) As the app I'm writing is pretty low-traffic (few thousand pageviews per day) I haven't noticed any slowdowns. In fact, the first thing that came to mind after your question was this article: http://blog.codinghorror.com/the-sad-tragedy-of-micro-optimi... :)

2) Nontechnical users will never read RDoc type code. They want things in plain English in the admin area where they do their work. I'm not sure which RDoc integration would provide that.