This is a case study of my personal experience on the performance of a site that I handle:
The site was ranking on top of the SERPs with a range of around 102,000,000 beating top competitors in the industry. I happened in such a way that I was forcing the site to rank on top for the other Keywords that have almost similar number and more sometimes in SERPs. This had lead me to think again when I learnt that the site is OVER - OPTIMIZED.
I realized this when the site literally dropped in SERPs from 1st to 8th position on SERPs for a competitive Keyword but not as much as the top competitive SERP's number. I was zapped and turned the archive on for the changes that I had done previously one after the other, then I came across a page....
.... this is none other than the page that was created new with the related content over optimized. This is a disaster for that poor term.