Edit Comment_Field while using T5 Comment Textarea On Top-Plugin

Change the textarea earlier than the plugin. The plugin runs with a default priority of 10, so you could use 9:

add_filter( 'comment_form_defaults', 'wpse_61103_change_textarea', 9 );

function wpse_61103_change_textarea( $fields )
{
    $fields['comment_field'] = '<div class="comment-field-wrapper">'
        . $fields['comment_field']
        . '</div>';

    return $fields;
}