@TheBigK - changing counts on new/future pages like that might make you feel like the site is less consistant but it might serve you better. You mentioned 400K pages in the G directory and almost 100K past errors, 25%. You could say that, theoretically, going from 10 to 20 posts per page might cause 50% of the site being 404. Is that pretty accurate?
You don't have to answer this question, but again, How much of the site's overall traffic LANDS on page 2/3/4/etc of discussions?