The session id shows on the forum at login and doesn't show when a user then chooses a forum or post to view as it's coded that way in phpbb.
Once the user is logged in the session id is recorded in the database and as long as the user stays logged in, there is no need to show the session id again in the url.
I use the SEO mod and it's brilliant. In the read me file it lists all the things that are changed and you can choose which ones you would like to use.
I would suggest you make a copy of your forum and put in another folder, then apply all the SEO mod changes to your forum copy and see what it does.
From there you can see what has been taken away and if you want to put something back just change the code back.
The last post dates and times stay with the mod, but it changes how they are displayed. The last post user name is changed to normal text and is not linked to the profile, the last post date changes to a link and the link is to the actual last post made.
From a search engines point of view it wants to see lots text content and focus on that, so the more images' and information that is not relevant to the page is removed the better.