You can also easily measure where the bottleneck lies. If your CPU is pegging 100%, the driver (or the app itself) is the bottleneck. If the GPU is pegging 100%, it's the hardware. Any recent tests I've seen have shown that the CPU usage with the FOSS drivers is not all that terrible (though not great) and yet the FPS is extremely worse. Clearly, then, the GPU hardware is the bottleneck; in this case, not because the hardware itself is bad, but because it's running in a race with its hands tied behind its back and one leg chopped off.