I haven't compared memory footprints, but 3.0b2 definitely feels faster. Fast enough, and stable enough, to have become my preferred browser over the past week or so.
Unfortunately, there are a lot of extensions that haven't been ported. However:
I tried one of the nightlies. They've really hacked the UI, but I'm not sure I think making it look more “Mac like” is an improvement. And whatever they've done makes cmSiteNavigation stop working, so I'm not gonna bother.
But I do recommend 3.0b2!
[Update 19 Feb 2008. Alas, b2 decided to updated itself to b3 and I lost both apps. However, the Tab Mix Plus forums lead me to a version that does seem to work in b3. The maintainer of the cmSiteNavigation Toolbar says he doesn't plan to continue maintaining it, so I guess that one's going to come down to me.]