It took me a long time to figure this one out, it is actually easier than it seems.
Open up wp-includes/post-template.php
Find the line that reads (it's inside the function named get_the_content):
if ( preg_match('/<!--more(.+?)?-->/', $content, $matches) ) {
And change it into this:
if ( preg_match('/<!--more(.+?)?-->/', $content, $matches) && !is_feed() ) {
And now you have partial posts on home page and a full feed!
