When I moved over a site to silverstripe the most important thing to me was preserving the existing link structure (http://www.silverstripe.org/migrating-a-site-to-silverstripe/show/251148). In the end I did this mostly either opening up phpmyadmin then editing the locations of the pages to whatever they used to be or using a hacked version of the nested urls branch.

You can see an example of this here -->http://www.rustyspigot.com/Computer_Science/Probability.htm - it is possible, but messy

I've tried to prevent duplicate content in silverstripe with 301 redirects in .htaccess
this can get quite messy, but is useful for redirecting domain.com/folder to domain.com/folder/

I'm also concerned about possible duplicate content in silverstripe (see my post http://www.silverstripe.org/general-questions/show/252604), though from what I hear google is kinder to duplicate content these days (its harder to do the old cgi proxy trick to remove sites from the index)...