Let’s say some reasonable scientist claims it turns out the speed limit of the universe is actually 8000 meters per second faster than the speed of light. That’s 0.0025% more. What experiment would disprove that? The only thing I know of is the Michelson–Morley style interferometer. I’m not sure what effect a slightly faster speed limit would have on the interference pattern. Besides, it still wouldn’t measure a preferred inertial frame of reference. There’s no reason to assume a preferred frame just because the speed limit turns out to be faster than light. (Right?) So I suspect it might have a very small effect. So what would it take to disprove such a claim?