So initial testing IQ is quite impressive. Tested with the 70-200mm f2.8 VRII at ISO 100 f4, shutter 1/3200 and compared with the D750 with the tripod in the same location, tripod same location with D750 in DX mode, and with the tripod moved closer with the D750 to try and match the original DX framing. Subject was a heavily embroidered Rossi cap. AF was performed using live view so there were no focus inaccuracies.
With the tripod in the same location, D750 zoomed to 1:1 and D500 zoomed to match the D750 at 1:1 the D500 is sharper IMO. If I then zoom the D500 to 1:1 I still think it's sharper, but that might just be perception as the subject appears closer. With the D750 moved closer to match the DX frame (although in reviewing I didn't quite move it close enough) I'm struggling to see any difference in IQ at 1:1 (just so happens that because I hadn't moved the camera close enough, at 1:1 'zoom'/magnification is pretty close), the stitching looks to have the same level in detail in both. It would seem that the lack of AA filter on the D500 makes up for the sensor size.
It was far from my most scientific test as it was done over lunch, but it was close enough for me to realise that the IQ of the 2 is very very close. Obviously as ISO ramps up I would imagine there'll be more of a difference. Also, in terms of overall reach I've discovered that 20mp on a crop doesn't offer as much more reach over a 24mp FF as I was expecting. Will be interesting to see how much difference this 'extra reach' gives me in the real world though.