There's already a bit delay built in.. for me fine, but maybe for reducing load on busy server an optional extra delay will be welcome.
Maybe for busy servers it's usefull too to limit the number of returned matches. (i got the idea that now all matches are showed)
My test case:
http://www.dokuwiki.org/start?do=search&id=plu
An annoyance for me is that the suggestions disappear when you type again in the search field. So when you were looking on the suggestions during typing, these were vanished. Not so helpfull.
Suggestion: don't hide the suggestions, and filter further by javascript the shown results, this can also reduce ajax requests .. till you recieve a new return from the next ajax request.
(This jquery plugin has some of these properties by using some caching
https://github.com/agarzola/jQueryAutocompletePlugin/wiki - as example to illustrate the idea)