Often we are asked when we move an existing site into Concrete5 if the site's search engine ranking is going to drop with the change to Concrete5.  Our experience has been that in most cases the keyword ranking after the move to concrete5 actually improves slightly.  One reason might be the dynamic XML sitemap that is created by Concrete5.  Another might be the search engine friendly URLs.  Lastly, it might be that the page load time is sometimes faster with Concrete5 than other content management systems.