No one knows what kinda of penalty this currently, if the Panda has a grip on your site it is not going anywhere.
I have been doing some testing with links and content. Google seems to be holding me back for the kw's that bring in the traffic. But for kw's that have low competition and no traffic I can rank easily and quickly.
I can send some links with the anchor text misspelled and within a couple days I am sitting #1 for that term. Do the same for the "keywords" that were hit by Panda and they do not budge.
They are setting the ceiling of how much traffic you can get on your Panda hit site and nothing is going to help get you past this ceiling until they decide to let go of it.
I am doing the same testing on sites not hit and they respond in the normal way thus they increase in traffic and move up in SERPS.