Selective file update

> Not really for saving download time but for not overwriting all files every .1 release….

Works in every aspect, Faster download of the zip, Less memory used for the download, Less memory/time needed to decompress teh archive, Less time required copying the files around..

> The generic string definitions file, like months, days etc will never be replaced so I can modify them happily, I hate the full localization….

That can probably be done via a plugin a lot easier, even if just using str_replace() on the gettext hook, for example: http://wordpress.org/extend/plugins/en-gb-localisation/ – It’s not the fastest method, but its simpler than a full translation. Even then, you can probably change what you need to change via a plugin just modifying $wp_locale.