Hi,
The change affects all drop-down fields, not just that single one. I checked the query, and the database returns 114 different manufacturers only within that field, even when the limit is removed. If the manufacturer information will be used on posts, then it should be available in the post_meta table, and they should start appearing in the list as well.
The file that was changed is: wp-content\plugins\ajax-search-pro\includes\functions\functions.php
in case you want to archive it.