Isn’t using anything other than the aspect ratio of a given HMDs displays wasting resources? So for example the Valve Index has a 9:10 per eye aspect ratio. If I use a 4:3 aspect ratio my GPU is rendering pixels that are outside the range of the display. If I want to get the best performance at the highest fidelity possible should I not set game resolution to a 9:10 resolution? Please correct me if I am misunderstanding something here.
In principle that’s correct. That’s pretty much what the ‘narrow’ resolutions in the custom resolutions suggestions are for, which (if applicable for a given game) are also utilized by the DirectVR auto resolution feature. In practice however 4:3 resolutions are a good way to keep things simple when it comes to manually choosing a FullVR resolution. Performance difference is often small or even neglible, not all games like uncommon aspect ratios, also with 4:3 you have a bit of headroom for ImageZoom and some wiggle room for vorpX’s custom frame interpolation. So all in all 4:3 is usually a good choice for FullVR without having to care about the factors above.
Depends on your preference, for example in fps games I like to use immersive screen and wide resolution which lets me feel separated body/head experience, I can turn head to see sides of the screen and when I move mouse around its not as confusing as with screen glued to face.
P.S.: missed you were talking of full vr mode, 4:3 is best for it and Im not sure you can use some hmd ratios since custom res means it must be supported by your monitor.