How to disable thumbnail filter for a specific template part or image size?

Probably the simplest solution is to remove the filter before output:

// remove the filter
remove_filter('post_thumbnail_html', '_sf_responsive_img', 5, 5);
// output template
get_template_part( 'something' );
// add the filter back on if other code below outputs thumbs that need the filter
add_filter('post_thumbnail_html', '_sf_responsive_img', 5, 5);