On Zwift it is hard to say, because it is a much larger group of riders than your local circle. If you are sourcing B's from around the world and 5000+ are logged in at any given time, it does not strike me as unreasonable to think that many who choose to race are capable of 300w.
In the real world? I have no idea. I'd wager most of the p/1/2s in this area are capable of it, and the ones that aren't are probably super lightweight. Below that it is a crap shoot.