Apologies to response to ancient post but I'm also looking for the answer. Switchresx's own documentation makes reference to files it alters in /System/Library/Displays/Override, even going so far as to suggest that you can sort the files there by date to determine which file was altered. But having installed Switchresx and created a custom resolution (that works!), none of the files there have a modified date of today. If anyone can shed light I'm sure many people would be very grateful. I'd rather not rely on preference pane add-ons for what is probably a superbly simple fix. Incidentally, I hear that you can use Switchresx to define a custom resolution and then uninstall it, leaving the new profile behind. But I haven't tried. Hope this helps someone.