The sensor is only half of the equation. The lens is the other half. The crop factor is a combination of the two.

Here is a method for proving it yourself. Take one DX lens and one FX lens, the focal length is not important. Use a white sheet of paper as a target. Take the lenses outside and first take the DX lens and holding it in you hand move it back and forth using the white paper in place of the sensor or film. Adjust the distance of the lens to the paper so that a subject at infinity is in focus and note the diameter of the circular image on the paper. Now repeat the same process with the FX lens and note that the circular image is larger than that of the DX lens. A camera's sensor size determines how much of that circular image is in your picture. The minimum image circle for an FX lens (sensor size about 24 X 36mm) is approximately 44mm; i.e., the diagonal measurement of the sensor. The minimum image circle for a DX lens (sensor size about 24 X 16mm) is approximately 29mm.

For a DX lens on a DX body the crop factor is always 1:1. For an FX lens on an FX body, the crop factor is always 1:1. When a DX lens is used on a FX body, the camera's electronics uses only the center part of the sensor (24 X 16mm) since anything outside that area is useless information. It is all mathematics.

😉

As noted above the original link was sold out. There are others at near that price so I'm guessing it is a legit gray market offer.

Still no inputs from anyone with that lens so I'll rely on published reports. They agree on good image quality at 300 mm.

Now I'm back to whether I really need it. I wanted it for trips by air where it's light weight and good image quality are assets. But I have other equipment that will do near that job and know that for most such trips I don't need a telephoto. And when I do need a telephoto (e.g. Africa) I'll lug the 200-500 anyway.

So I'm holding back the GAS.

The image circle is different of course, but does not make any difference in the crop factor.

The image circle of an FX lens must be larger than the fx sensor, and the image circle of a DX lens must be larger than a DX sensor if not then the result is less or no light at the edges. If an FX lens is uased on a DX camera, no problem because the sensor only gets light from the center, a DX lens on an FX camera will provide light to the center of the sensor, but not to the edges, unless of course the camera has the electronics to put the sensor into crop mode. (Not all cameras have that option.)

The crop factor though is still based on sensor size as it determines field of view.

The image circle is different of course, but does not make any difference in the crop factor.

The image circle of an FX lens must be larger than the fx sensor, and the image circle of a DX lens must be larger than a DX sensor if not then the result is less or no light at the edges. If an FX lens is uased on a DX camera, no problem because the sensor only gets light from the center, a DX lens on an FX camera will provide light to the center of the sensor, but not to the edges, unless of course the camera has the electronics to put the sensor into crop mode. (Not all cameras have that option.)

The crop factor though is still based on sensor size as it determines field of view.

Me too. It is the ultimate in light weight and features: Panasonic Lumix GM5. I have a 45-200 lens for it, which is 400mm equivalent due to the smaller M43 sensor. But it is only 16MP in 3:4 image format and 12MP in 4:6, and has inferior high ISO performace than my D5300.

My D5300 takes noticably better pictures with it's larger 24MP sensor. And perhaps more importantly I am much more capable with its features and controls. While the Lumix actually has superior features (e.g. Panos, touch screen control) the controls differ from Nikon and I haven't learned them fully. So when size and weight aren't as compelling I'd rather take the D5300. I can use my 28-300 on it, which is 450mm equivalent. Although for city and landscape trips I prefer only my Sigma 17-50 f2.8.

When the trip is primarily for wildlife (e.g Africa) I lug the D800 and 200-500.

I have similar experiences with my M43 with one difference. I find that my M43 takes great pictures in bright light. In bright light, there is a slight difference in color quality between the M43 and my D7000, that can be seen when viewed side by side on a good quality monitor. But with pics taken in bright light, the difference it is slight. The casual viewer would probably not notice it at all. But I'm not sure it's at all a fair comparison as the two are not even from the same generation of sensors.