You are off by a factor of 2 in your pixel calculation, because 5k x 5k is for a stereo pair of spheres. Equirect projections waste a fair amount, but compared to the 300% miss to get to 60 fps stereo, it isn't dominant.
Anything you can do about someone tilting their head? That feels like the biggest remaining immersion breaker with pre-rendered stereoscopic videos.
Starting where? Turning where?
> However, if you tile like that with 5120 x 5120 you get a lot of wasted pixels at the poles.
You didn't mention tiling.
> If a 5120 pixel Radius is fine... then Radius of sphere = 2 pi * r, and Surface of sphere is 4/3 pi * r^2. so 5120 / 2pi =r => 4/3 * (5120/2pi)^2 => (5120)^2 /(3 pi) ~= we need ~10% of 5280^2.
More details on what you are trying to say at each step please.
As to tiling, I am saying if cut a sphere into 5120 slices, and the middle slice is fine with 5120 pixels. Then the poles are going to have ~1 pixel on them. How you tile them is really a question of tradeoffs.
> As to tiling, I am saying if cut a sphere into 5120 slices, and the middle slice is fine with 5120 pixels.
What kind of slices? Like sections of an orange? Like longitude?
If a 5120 pixel Radius is fine... then Radius of sphere = 2 pi * r, and Surface of sphere is 4 pi * r^2. So 5120 / 2pi =r substitute for r > 4 * (5120/2pi)^2 simplify > (5120)^2 /(pi) ~= we need ~1/3 aka 1/pi of 5280^2.
However, I suspect you actually want more than 5120 pixel radius.