Here is some data.
Slightly different application, bottom brackets, but same question: Is ceramic better than steel. Top of the line ceramic threaded bottom bracket generates 0.31W of resistance. Top of the line steel threaded bottom bracket generates 0.32W of resistance. That’s Jason’s data (which I paid for but is now available for free on the internet…I’m so smart). And yes, he can really measure 0.01W.
So while it’s techincally correct that ceramic is better the improvement is de minimus and on a $/W basis, astronomical.
Ceramic has a little more advantage in a wheel because ceramic performs better at higher RPM. Let’s be honest, though, even on the fastest downhill, RPM in a wheel bearing isn’t that high. Ceramic probably has some slight resistance advantage and might weigh a little less but neither advantage is worth the cost IMO. I’d bet that the difference between a quality ceramic wheel bearing and a quality steel wheel bearing is less than a couple hundred milliwatts.