GreatBear, even if the plane had power and was more or less under control until it's last ACARS message I suspect it could not be outside the 40 mile limit after a controlled glide into the water.
For that matter given its momentum I bet you could describe a figure that is slightly smaller than a simple 40 mile radius. Such a figure might go backwards along the flight path as little as 20 miles.
If BEA figures out a sane figure of that sort it might reduce the expense of a thorough search somewhat.