I am excited to finally add my 2 cents to this thread.
I've seen a little bump in rankings (I don't know if I'm a prawn or not-- my paid books rank around 100ks).
My free romance novelette is ranked just barely in the 2ks when it was 3ks earlier today and I haven't had an increase in downloads.
The other two books attached to the first went up like 10-20k spots but are still in the 100ks.
I noticed something else interesting-- my ams budget took forever to spend completely today which is unusual. It's usually spent just after noon (my budget is small and CPC is high because romance) and it wasn't spent until nearly 8-9 pm here. I don't know what amazon is doing, but they need to leave well alone.