I was a bit surprised, and this renews my interest in the D300 as a useful camera in my arsenal.
Here's the scenario. I mounted my old Nikon AF-S VR NIKKOR 70-200mm f/2.8G on a tripod and tested my D850, D500, and D300 with the FX camera set to 160mm and f/8 and the DX cameras set to 105mm and f/5.6, all three at base ISO and the same shutter speed. After converting the NEF files to TIFF with NX-D and opening them in Adobe Photoshop I then applied Smart Sharpening to all of them (same amount for all three) and resized them to 2160 pixels (vertical) to see how they would display on my 4K monitor. Note that Picture Controls using NX-D have changed from the days of the D300 to the D500/D850, so I used the same Picture Control on the latter two and an older D2X emulation Picture Control on the former.
Anyway, here's what I see, and it should be self-evident looking at the 100% size crops at the bottom which camera was used in the re-sized to 4K crops directly above them:
This doesn't discount the usefulness of the D500 and D850, both faster and with more features than the D300 (or my D800 for that matter), and if one wants to start cropping away then the 100% sized views become more relevant. That said, IMO there will be a lot of times when the old D300 can hold its own.