Hmmm, interesting. That may be true, I don't know. However, I'm pretty sure we only see relativistic effects when something with mass approaches the speed of light in a vacuum.
True; relativistic effects involve relative (!) speeds of ~300,000 km/s. Only massless things can move _at_ that speed. Nothing moves faster.
As observed in water-based nuclear reactors (glowing-blue photo in original article), if light is slowed by traversing some medium, some things may be able to move faster than light _in_that_medium_. This does _not_ violate relativity, as it’s the ~300,000 km/s that matters, not the slowed light. When something does manage to go faster than medium-slowed light, there may be an effect akin to a sonic boom (thing making the noise is moving faster than the sound itself; likewise, thing making the light is moving faster than the light in that medium).
Kinda weird.