including post-specific feed without full wp_head()

all wp_head() does is run all the actions attached to it:

function wp_head() {
    do_action('wp_head');
}

you can call get_feed_link() directly.

however –

it’s important to note you’re going to break most plugins by not calling wp_head() a better route may be to just remove_action on all the things you don’t want in wp_head(). look in wp-includes/default-filters.php for all the actions attached to it.