As long as I'm suggesting small improvements from the MCP, how about adopting the keyboard shortcuts they added to the spell window. Basically, type a letter and selection will change to the first spell of that name in the list.
As long as I'm suggesting work for someone else to do, there's a couple of things the MCP doesn't do which would be useful.
The first is scrolling so that the newly selected item becomes visible.
The second is to either cycle through all the entries starting with a given letter on repeated keystrokes, or else adding some sort of incremental filter. I'd suggest the first approach since I always find Gnome style filer filters to be slightly clumsy to use for some reason.