I would do them any maintenance on one table at a time, as if one errors out you'd be more likely to pay attention to any associated error.
Zlib will compress the output of pages sent to your users, they would SEE things faster, but it would still take the same amount of time to execute any queries.
Isaac's additional code simply runs the trigger every time someone logs in, in addition to just every time a post is made; you'll notice it "as time goes by", it won't show immediately, perhaps after a couple of days of users using the forums on a normal basis.
The UBB.Sitemaps and UBB.MemberMaps scripts only really aid in more bot traffic for search engines that support them (such as Bing and Google), other bots would have to be told about where the sitemap is to be able to use them; but after you've provided them with the url to your sitemaps they'll process them as needed, you can tell them to reindex them through either Google Webmaster Tools or the Bing Tools centers.
Any filters on the machine could add to additional time (just like a firewall on a computer would add to overall usage); you'd have to play with it to decide if it'll be faster for your server; the more available resources that you have available, the faster everything is going to be able to run.
