Just ordered two of these myself. I'd be happy with getting any mpg gain at all, but 2mpg is nuts. Fingers crossed that this will put me above the 15mpg mark...
My mileage was getting really bad over the last 10k, which I now understand is a common symptom of aging O2 sensors. It's been only a few weeks, but I am getting about 12/16 instead of 10/14.