I have 50+ mail filters set up, and it's always a pain trying to modify an existing one, as it takes ages to find it-- the ability to search on the filter name isn't usually sufficient.

Two possible UIs spring to mind (which could co-exist):

- Modify the "Create Rule" section of the Message menu/context menu to include "Show Rules for this Message", which would show you a list of all filters that would apply to that message (whether or not, in practice, things like 
"Stop Processing" directives would cause them to be /actually/ applied, although you might consider flagging those somehow.)  If there is only one such filter, just open it for editing.

- Add a proper search box to the existing Message Filters dialog, that filtered the list of filters(!) as you typed into it.  So if I typed "jds", I'd see a list of all filters that matched mails from "jds@sun.com", or filed messages into folders called "jds-ui" or "jds-dev".

All the mail clients I use (Evolution, Thunderbird, Mail.app on OSX) lack this feature, and I curse them all for it regularly!!