That's a lot of pages to leave untouched - and presumably not all of them perform well - some might. Did you consider cutting pages with poor performance, low PR , high bounce rate?
Seems like you're playing with a threshold, that needs further major fixes.
Also the reason i asked about your site type is because i have a hunch that Google is hitting sites in competitive verticals . It must be, because the content has relative value , and add to that Google's own self interest in occupying certain verticals with it's own properties.
( I am seeing this update as not perfect from my limited view. One set of key results brings back 4 results from the same "branded" site in the top 10 organic results of a couple of popular phrass. So it may be a combination of things.)
[edited by: Whitey at 1:34 am (utc) on Oct 1, 2011]