Comparing sensors based on size alone is a nonsense.
Firstly, resolution is a combination of lenses, megapixels etc. Shallow DOF is nice, but if you want more DOF, APSC is easier to live with.
However, the DR of current generation sensors is hard to compare. Check out the Nikon D7200 on DxO against the D750 and Sony A7II (both 24MP). Very little difference at low ISO between the two Nikons. You won't notice this in practice. At high ISO, D7200 falls a bit behind the D750, but not the Sony.
Both better than the Sony. So much for sensor size.
Now compare the D7200 with the Nikon D4. Smaller sensor is notably better at low ISO, but about a stop worse at high ISO. Which is best rather depends on where you need your DR.
Now compare with Canon 5D3. D7200 is nearly 3 stops better at ISO100, and doesn't drop behind until ISO12800.
Sensor design is just as important as size.
If DR is poor, this will also affect tonal separation in the shadows, but overall tonal range will be better for a larger sensor, and so will colour performance. That's simply a higher SNR in mid-tones and highlights. That also implies noise will be less visible, at that will be more obvious at higher ISO. However, at low ISO you won't see much difference. All of them produce more tones than a screen can display, and more colours than you can see. The noise is not likely to be seen in a print at 250PPI or more, at least up to ISO400.
Is there some point in FF sensors? Sure, if you want shallow DOF more than high DOF, and if you want to shoot primarily at high ISO (ISO800+).
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"Don’t put your trust in revolutions. They always come around again. That's why they’re called revolutions" T. Pratchett, OBE, RIP.
I'm not sure what you are saying regarding "if you want more DOF, ASPC makes it easier to live with." Are you suggesting stopping an FX lens down further will significantly increase sensor noise and lens diffraction issues? If as you say, the increased noise isn't visible in 250PPI prints, why would diffraction issues be more apparent? Sorry if I am misinterpreting your ost.
I am saying that if you shoot high DOF images (landscapes for instance) then DX allows you to use a wider aperture and higher shutter speed. If stopping down means raising ISO (a lot of the time my D800 is at ISO200 or 400 just to keep the shutter speed high enough) then you lose any advantage from the FF camera. Unless you use a tripod, but that's a disadvantage in itself.
So, if you seldom shoot above ISO800, or seldom need razor thin DOF (which is highly overrated) then FF is not buying you anything. Unless of course, you need more than 24MP and don't have any high quality DX lenses.
The biggest issue with all DX SLRs from Nikon and Canon is lack of decent dedicated lenses. Cropping the image does lose resolution.
On the other hand, you have less need of an AA filter, simply because of the smaller pixel pitch.
I would seriously recommend looking at the IR comparison of the D750 and D7200. See if you can spot any difference up to ISO 400. Download the RAWs if you want. It's a wash.
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"Don’t put your trust in revolutions. They always come around again. That's why they’re called revolutions" T. Pratchett, OBE, RIP.
D810 is tops, mainly due to having a 64 ISO base. The D810 has 14.8 EV... next up.... D7200 DX with 14.6 EV which beats even the D750 with just 14.5 EV.
