In ACF5 there is a new filter in the user field that can be used to alter what is displayed, which is not documented. acf/fields/user/result.
You can see them at line 304 of /fields/user.php
// filters
$result = apply_filters("acf/fields/user/result", $result, $user, $field, $post_id);
$result = apply_filters("acf/fields/user/result/name={$field['_name']}", $result, $user, $field, $post_id);
$result = apply_filters("acf/fields/user/result/key={$field['key']}", $result, $user, $field, $post_id);
$result => value to be displayed ie, user name
$user => the user object
$field => the field object
$post_id => the current post id
add_fitler('', 'change_user_display', 10, 4);
function change_user_display($result, $user, $field, $post_id) {
// change $result to what you want displayed
return $result;
}