A couple small suggestions to combine with the "show new posts since last visit" page:
It would be useful to have a "visited link" color that distinguished the threads you'd clicked on from those you hadn't clicked on so you know what you had already looked at. Also, at the bottom of a thread (after you've already read everything), a nice little "back" button to the lower right might get you back to your navagation new posts page more quickly.
My database is somewhat large at this point, maybe 300mb. I chopped off 100mb by deleting the top 300 most common words from the search tables (the version I was using I never caught the error until recently). I haven't installed a test version of this board but am interested in finding out how you've structured the search table. I hope it still works well as it gets larger.
By the way, the hacks I love for phpbb that I'm thinking may use too many resources are navagation hacks at the bottom of the page that go to the next unread message, somewhat like your new posts since last visit page but on the thread itself.