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So your asking can you mix up rss data to try and make it look unique in Google's eyes and benefit from it. It will be seen as duplicate and when you have the rss links point back to the resource im sure Google will work that out.

Yes, I agree the ideal solution would be to make the content unique, however all being well, I will have about 20,000 threads and 50,000 posts added in a month. The other main reason for doing is it the forum script creates users as assigns posts to them so the forum will also seem to have about 5,000 active users. Removing the duplicate content would be easy enough, can run an sql query and remove all posts before x date,

Ha, I agree, though I think it gradually starts skewing in one direction.

I dont think you read my comments correctly. I never said you can not ranik with CMS only that they spit out violations, obvioulsy you would do better without them. for example your site has over 18,000 violations according to Bing over 5,000 broken links over 4,000 html errors over 800 unessasary redirects leaking link juice 800 cases of long querry strings over 200 canonical issues I will have to say that your answer is based on assumptions that joomla was good for SEO