Well, theres a couple of hints for you... You should limit your sitemap to 25k urls per page, use a sitemap index if you need to go over.
Theres no need to have your users or posts in a sitemap, all you need is the topic link as the posts will all go here, you'll just be feeding it more data than it needs.
A sitemap won't get all of your stuff crawled overnight; you're simply aiding the SE's discovery bot in finding new content, it can still take months for all of that yummy content to get indexed.
A huge sitemap will cause strain on your server, the sitemap index will definately help ease requests, especially if the sitemap itself isn't cached and builds live, you'll be putting strain on the sql server for having it push out all possible urls.
Also, try avoiding duplicate URLs, once it has a url it'll crawl all possible urls on that page, feeding it say 5 possible urls to the same page is simply going to bog down the request time...
