Golite used to have something like that. Now they have only this: http://www.golite.com/About/why.aspx?e=0 Bottom line, weight goes down, speed, mileage, and fun goes up!
I absolutely believe that there is a clear relationship between weight and speed and also weight and mileage. I have loads of qualitative data (people's stories) and my experience to back that up. However, I think there are too many variables in the equation (such as fitness, nutrition, elevation, attitude, weather, etc) to have a clean 1 to 1 correlation (at least in a quanitative sense, which would create a simple numerical relationship), at least in a truly scientific sense.
BUT if you find something that's clear, I'd love to see it!