I now have the Tokina 11-16mm f/2.8 AT-X 116 Pro DX II lens.

I walk outside and ran a quick test and got almost the exact numbers you did. I used the eye dropper to make a more precise measurement.

At f5.6 I measured a .56 difference in exposure and at f16 measured a .73 difference in exposure. I shot a grey door, and measured in the middle to avoid vignetting.

hmmmm