Given the reversal of the image through a lens, what constitutes the "top of the image" is really the bottom edge of the sensor. Because the mirror clears the bottom of the sensor first, and the top of the sensor last, I am positive that it is not a mirror issue if your image has a shadow on top.
At speeds up to 1/250ths of a second, the whole sensor window is open simultaneously. Anything faster, and the shutter opening is only a fraction of the image. At 1/500 of a second, a slot, covering 50% of the height of the sensor, is moving from the top of the sensor to the bottom. Confusing as this may be, the slot starts at the top of the sensor, which is the bottom of the image. It ends at the bottom of the sensor, which is the top of the image. At 1/1000 s, the slot covers 25% of the height. At 1/8000s, this is reduced to a slot of just 3% that races through the sensor field in 1/250 of a second.
Now: if you have a shadow at the top of the image, something strange is happening in the last microseconds of the shutter actuation, when the slot arrives at the bottom of the sensor (= the top of the image). A shadow would indicate underexposure. This would suggest that the slot narrows on the last few millimeters of its travel. We can therefore be reasonably sure in saying that the leading curtain is slowing down prematurely, while the trailing curtain keeps its speed to the end.
I think this is a warranty issue. I know that the issue is complex, and I can't describe it in simpler terms. But Nikon's service department should not have trouble with this reasoning and should give you a free fix or a replacement. Good luck!