Ajax on textfield keeps me from typing
Your best bet is to either 


use the #autocomplete_path and tweak the behavior by overriding the JavaScript (prototyping)
Don't use the ajax framework, but create this with custom JavaScript.


The AJAX framework can do a lot of things for you, and works very great for most things, but there are some cases like this one, where it doesn't work as intended. The problem is that you will get too many ajax requests at once if you do one for every keyup event, which most likely will mess things up if you could actually do it. This is why you need an autocomplete functionality which usually caches the results for improved performance.