From all I know none of the systems that people have built come even close to testing the speedup: Is error correction going to get harder and harder the larger the system is, the more you ask it to compute? It might not be the case but quantum uncertainty is a thing so it's not baseless naysaying, either.

Let me put on my tinfoil hat: Quantum physicists aren't excited to talk about the possibility that the whole thing could be a dead end because that's not how you get to do cool quantum experiments on VC money and it's not like they aren't doing valuable research, it's just that it might be a giant money sink for the VCs which of course is also a net positive. Trying to break the limit might be the only way to test it, and that in turn might actually narrow things down in physics which is itching for experiments which can break the models because we know that they're subtly wrong, just not how, data is needed to narrow things down.