[Mach1Canuck]

By 'Your mileage may vary'. I just mean that you could have different results than me..
I haven't figured out the mileage though, but I am speculating that it will increase slightly, but not enough to really measure.

If you think about it, if it idles lower/returns to idle faster, than it uses less gas right?

Also, you use less gas when you don't want to (letting off throttle, braking)

This is all assuming the engine computer will compensate for the richer condition caused by less bypass air.
Hopefully, it looks at the O2 sensors and lessens the amount of fuel being admitted.