No one "comes" back from anything post penguin. I have yet to see a site return to anything close to their previous positions, no matter how much they spend.
I agree, I've got a ton of sites in my portfolio both personal and client based. I'm having a hard, hard time regaining what was lost with some sites. Even when the pundits say Penguin targeted x,y,z - the truth of the matter is that there was some random shit that got sluiced down the net turnpike, and good/bad sites got hit for apparently whatever trigger they set off off in the main production link inidice database Google serves out to the public.
For many of my clients, I''ve gone ahead and created satellite sites that will act as the ranking authority site and then just keep their old site as the reference site for business reference. It's far easier to create new sites and rank those, than to figure out what penalties were levied and why a site dropped off the face of the SERPS. Yea, we can identify the lack of title link diversity before penguin, but post penguin repair seems a lot harder that it has been before.
Oh, and don't be fooled that just because your links aren't getting production serp INDEXED, that they're not getting spidered and placed into the pre-prod sort index and recognized. Far to many people think of Google has this huge index of deliverable information, when in reality (and I know first hand for many reasons), it's a series of indexes that have cross relationship index pointers to feed what we see as the main index production that gets served up when you go to google.com
Penguin, to me, is about key identifiers which single out sites as being overly aggressive to manipulate website ranking in the serps. So yes, negative seo works depending on the level of authority your site commands. You want to be evil and do a case scenario? Forget 20k profile backlinks daily, go get 10k backlinks from known bad XXX porn communities and watch a site tank faster than a senior going down on you at prom night. This was standard REAL blackhat SEO stuff back in the day to manipulate rankings. I never did it, but know far to many people who did. The same type of folks who built doorway pages based off of php scripts to single out keyword search hits from google and serve up cpa or adsense ads.