Overscan doesn't remove the black borders.
It just extends it to fill the grey space made by the Amiga.

Do you get the same black borders that can be seen in this photograph? They do actually look bigger than they are in the photo. The mode used is MultiScan productivity. I used the vertical knob to sctrech the picture to go right from the top to the bottom, and therefore elimintae any black bars at the top and bottom. There is no horizontal knob to stretch it the other way though (unless there is one inside)