@blindmikey We’re going to do something similar using the Local JSON feature, which I believe was added in ACF 5 (in Dec 2014?):
http://www.advancedcustomfields.com/resources/local-json/
Each field group gets saved into its own file, so you never have to deal with large files full of fields. Unfortunately the JSON filenames aren’t very friendly, but the workflow is so clean (since 5.3.1) that you shouldn’t need to edit fields manually very often anyway.
Also see:
http://support.advancedcustomfields.com/forums/topic/the-acf-json-workflow/
