I tried this on a few rental Volvos: S40, S60 (2005-2008). I think it starts it in 2nd gear, as mentioned -- but (if equipped) the electronic throttle is muted -- so if you mash the pedal, it gradually accelerates. Shifts are softer as well, so it doesn't break loose during gear engagement on slick surfaces.
I used it for a bit and determined that it just wasted fuel between shifts. I would start in normal mode, press the "W" button, and let it be a bit more gradual in how it operated. IIRC, it shifts sooner so you could be fighting the torque converter and get that "working hard" feeling.
I have no official data -- I would recommend to do a test, A-B-A style and record mileage on each run. You would have to reset the car's MPG each time or use a
scangauge if you have one -- that sort of thing. Try to maintain consistent pedal pressure on acceleration, maintain the same cruise speed, brake in the same spot, etc. Figure a small route with little traffic if possible and run the same direction.
Best FE to ya...
-RH77