Can you improve performance when moving from a statically generated site to a dynamic environment? You can if the conditions are right. In the case of CDT, publishing times were a nightmare with Movable Type. Search performance was horrible, and the comment spam problem caused such a drag on the server that we’d had to disable commenting altogether. Now, with the site fully tag-enabled, searchable and comment-able, loads are down dramatically and publishing times have dropped from 15 minutes to a few seconds.
Mark has a great post about the release of the GPL’d Moveable Type and the differences between it and WordPress. Welcome to fork city!