Make it possible to customize the built-in AsyncFilteringBehavior by inheriting it or it's interface
Add it on your wishlist X 2 :)
This would be very nice. With large sets of data, doing the filtering on another thread is a must.
The following forum thread discusses how such custom AsyncFilteringBehavior could be created in the current version of RadAutoCompleteBox: http://www.telerik.com/forums/dropdown-speed-display-problem