Now that the OculusMirror tool is available, we can see the final image that is sent to the hmd after all the distortion steps. So we can now capture the actual effect of the pixel density setting in the debug tool.
For this I've used Elite Dangerous. It lets you change the pixel density within game without the debug tool. This is useful because you can't change density with the debug tool while a game is running, and restarting Elite moves the camera so the shots wouldn't match.
(My CV1 is sitting on a stool with a piece of foam holding the proximity sensor active)
Here's the Elite Dangerous hangar menu scene in 3 different density values, with layer debug panel on to see the res and density.
0.5 density (672x800 per eye):
1.0 (1344x1600 per eye):
2.0 (2688x3200 per eye):
Of course in all cases the final image is still 1080x1200 per eye. It's just the render texture used to render the scene and give to the sdk that is a different res.
Here's some zooms to look at the difference a bit easier. In each one, it's 0.5, 1.0, 2.0.
I tried bumping density to 4.0, but Elite freaked out and dropped all environment textures to horrible blurry messes.
Performance wise, even with a Geforce 1080 a density of 2 gives tons of dropped frames in Elite. It's trying to render at a total of 5376x3200 after all, well above 4K. There's a heap of settings in Elite that could affect that, but I'm already running with most at medium quality.