This allows for breaking of post content in feeds at the more tag, rather than relying on the excerpt.

feed.php

<?php
/**
 * Customise feed output.
 *
 * @package Themename
 */

/*
 * Break feed content at the <!--more--> tag.
 * "For each article in a feed, show" should be set to "full text" for this to work properly.
*/
function themeslug_content_feed( $feed_type = null ) {
	if ( !$feed_type ) {
		$feed_type = get_default_feed();
	}

	global $more;
	$more_restore = $more;
	$more = 0;
	$content = apply_filters( 'the_content', get_the_content() );
	$more = $more_restore;
	$content = str_replace( ']]>', ']]& gt;', $content );
	return $content;
}
add_filter( 'the_content_feed', 'themeslug_content_feed' );
add_filter( 'the_excerpt_rss', 'themeslug_content_feed' );