One other thing you can do is look at your weblog stats and see what pages get it the most, you usually can get away just building a few static pages (initial site drop in and main sections) to achieve a substantial performance increase. Building all of the pages as static on a schedule can actually decrease performance if you have a lot of dynamic pages that do not get used frequently.

If you must build the entire site statically and there will be 15k html pages, it may make sense to make a logical tree so stats on the files do not take as long. if you split those files up into 100 -> 1000 file/directories you should see a nice performance boost on stat (on most filesystems).

After all is said and done, think carefully about doing this, by statically building pages you lose the ability to customize based on login/region/etc -- static is by definition not dynamic. =)

You may also want to take a look at mod_perl and some of the DB caching modules out there -- you may be able to squeeze more performance out of your box that way as well.