I purchased and installed the VOLO VP12 soft-flash chip 2 weeks ago on my Miata 2002 1.8L base model.
The car also has a K&N short ram air intake, which of course also adds a tiny performance gain and some small mpg gain of its own.
After doing 230 miles on 6 gallons of gas, I am a believer....almost 38-39 mpg now. The power is also perceptibly more.
Not sure if this is going to help you, but at least now you know the product does work.