The Sigma 50-150 is a DX-only lens. It will not light the full frame.
That means you have to use a DX camera to compare it with any other lens. The D7000 is the best performing DX camera that I can see test being availabe for the 70-200/f4 so using this camera was the best choice in my opinion.
The 28 points you're referring did the 70-200/f4 earn on the D88 which is full frame.
Hope that helps.