Different post count on each additional page (i.e. 9 posts on page 1, 6 on each additional page). Does some recalculations WP needs in order to get the page count right.

functions.php

<?php

$tbs_per_additional_page = 6;

function tbs_paged($query) {
	
	global $tbs_per_additional_page;
	if ( $query->get('post_type') == 'post_editorial' || $query->get('section') != '' )
	{
		if ( intval($query->get('paged')) > 1 )
		{
			$offset = intval(get_option('posts_per_page',9)) + ($tbs_per_additional_page*(intval($query->query['paged'])-2));
			$query->set( 'posts_per_page', $tbs_per_additional_page );
			$query->set( 'offset', $offset );
			$query->set( 'showposts', $tbs_per_additional_page );
		}
	}
	
	return $query; // Return our modified query variables
}
add_filter('pre_get_posts', 'tbs_paged');

function tbs_posts($posts)
{
	global $wp_query, $tbs_per_additional_page;
	if ( $wp_query->get('post_type') == 'post_editorial' || $wp_query->get('section') != '' )
	{
		if ( $wp_query->max_num_pages > 1 )
		{
			$wp_query->max_num_pages = intval(ceil((intval($wp_query->found_posts)-intval(get_option('posts_per_page',9)))/$tbs_per_additional_page)+1);
		}
	}
	return $posts;
}
add_filter ('the_posts', 'tbs_posts');