Since DokuWiki caches it's content (like MediaWiki does) you probably won't notice any differences regarding the "delivery" of the pages via the webserver. The only bottleneck might be the search/index. Since these are flatfile based it's a little bit slower than with a database backend. However, the search/index mechanisms of DokuWiki have been improved during the last releases. And I think 8000 pages is far from the limit before you get into trouble. There are also plans to move the index/meta data of pages into a sqlite database once the switch to PHP5 happened (which contains sqlite by default).
Maybe someone can post some numbers (I don't know how many pages are on the
http://wiki.splitbrain.org wiki but it's a lot for sure).