The "fact" that Penguin requires intensive computation coupled with other stuff I'm seeing leads me to the hypothesis that some form of ratios are involved. If you are at, or just past, the tipping point then working to correct on page/within site over optimisation OR backlinks/backlink anchor text should have a positive effect.
If you are in this situation, as I believe I am, then as I've eluded to earlier it may be best to focus on your own site first, then sort out any slightly dodgy backlinks or anchor text.
I'm worried that hard won backlinks with KW anchor text may not be the thing tipping me over and once I get rid of them I might do irreparable damage.
If you are badly affected there may be no alternative but to aggressively do both on site and backlink anchor reduction. In this situation I'd like to bet you already know why you've been hit and can relatively easily decide what you are going to do.
I've found it most telling to analyse competitor sites that have been affected either positively or negatively. It is easier to understand why those that have been hit got hit. Those that have benefited are harder to understand as some are similar to sites that were hard hit.