Let me answer these in order:
(1) Has the letter worked? It depends on what you mean by "worked"? Does Getty roll over once they get my letter and settle for $100? No of course not. But the letter does stop them from communicating with you directly and putting you "in collection" with NCS Recovery, their "collection agency." It also provides a feeling of community in that you are not fighting Getty alone and are joined with dozens and dozens of other businesses that have tried to address their concerns in this fashion. We are not communicating with Getty at a high level. Forums all across the world have referenced our approach and are watching to see what will happen next with Getty. On one popular forum, there are discussions about engaging Getty in a global settlement discussion of the hard issues in the case. But the main benefit of the letter is having their communication cut off and the endless chain of emails and letters ending.
(2) How much lower have they gone? - To my knowledge the lowest I have heard them accept is around $800 per image. Someone reported to me they accepted $500 per image but I have not recovered any proof of that.
I do not think they will likely sue a single image user so you have the option of ignoring it and seeing what happens.