Ok, we will do a little experiment. Right now my site is getting 2.3 - 2.4 million page views a month, a steady 13% - 16% growth every 30 days (its been that way for months), 340,000 urls indexed with google and my site holds several #1 spots in google SERPs. I turned off the show post submission on the site map settings. And lets see what happens. I'll keep this thread updated. But if the growth slips below 10% every 30 days I will turn the showpost setting back on. My site has not been below 10% growth every 30 days in months. So I think that would be a good measure and cut off point. Also, I have held those #1 spots for around 8 - 10 months, maybe a year. If I lose that ranking the setting will get turned back on. But, there is another keyword I have been watching. My site has never been able to get past page 3 for that search. I'll keep you updated on that keyword as well.
Also make sure you've got heading tags for your pages. You can use CSS to style them to better fit your site instead of having huge H2 and H3 tags throwing everything off. If you've done any link building, remember to attempt to keep the same amount of effort into it. Use a good robots.txt to block the remainder of the duplicate content And without knowing your URL, ensure its only accessible via either the www.sitename.com or sitename.com version. And update your preference in google webmaster tools. This can be done with a 301 Redirect on .htaccess I'm interested in the results of this test as well And hope it helps improve your site.
I did the htaccess thing a long time ago. All traffic is pointed towards www. webmaster tools has been set to www as well. Here is a screen shot of my traffic for the last 30 days. According to Google Analytics my site has had 15.61% growth in the last month. My site map will update tonight, but its might take a week or two for all the changes to go through. I'll update this thread in a week or so.
Ok, here is an update on the results - my traffic has dropped over the past 3 - 4 weeks by around 4% - 5%. For almost the past year, my traffic has been at a steady 13%-16% growth every 30 days. The highest its been since I changed that setting was around 14%. The traffic has dropped at a steady rate of about 1% per week. This is the first time in MONTHS my forum growth has dropped to less then 10%. It currently stands at 9.53%. When I started this thread I stated that if my growth dropped to less then 10%, the experiment would be over. What I am going to do now, I'am going to start submitting the threads in the site map once again. In 3 - 4 weeks I will post the results.