In the center of the lenses, the circumference resolution is a bit over 5120, but definitely less than 5760. Even at 5120, it is a bit overkill (and potentially aliasing) at the edges: https://twitter.com/ID_AA_Carmack/status/975198157838499840
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.