Doing 4 8-bit computations in parallel is different from doing 3 10-bit computations in parallel (or two 11-bit computations and one 10-bit computation). The hardware required is different.
Modern GPUs are certainly capable of high-precision arithmetic but it is not a safe assumption that all the hardware is designed for it. A lot of it is designed for the stuff it spends most of its time doing, which is 8 bits per channel rasterization.
Modern GPUs are certainly capable of high-precision arithmetic but it is not a safe assumption that all the hardware is designed for it. A lot of it is designed for the stuff it spends most of its time doing, which is 8 bits per channel rasterization.