Brian - you're probably right about 1200 pixels. As a web post viewed at 100% that is near full screen, but at 300 PPI it's only 4 inches on the long side. At that point details go away from both images.
Now the 10 inch metric is pretty close to the 8x12 inch size used by DxO Mark for their sensor analysis. So I think at 10-12 inch prints and larger you will notice detail.