I am not sure what races you are doing in Zwift but the EVR ones appear to be pretty legit, and the single west coast one I did was much more civilized than the East coast EVR races I normally do (i.e. the first few minutes aren't all out). I am towards the front of the Cat B races (3.9 W/kg), but obviously I don't race as intelligently as some others as I generally get beat by people putting out both lower total watts and lower W/kg. Although on the flatter courses the big guys come into the equation. Anyway, these lower cat races are an excellent workout/ride/race and almost everyone has either a smart trainer or power meter (can't actually remember the last time I saw a zPower dude in the results...maybe they're not permitted in these EVR races?). The cat A are another beast because there is obviously no upper limit, although if you go above 5 W/kg you have to provide real world results to back it up, which I think is very fair.

BTW, I will increase my weight to stay legit in cat B races (i.e. so I don't get DQ'd for going 4+ W/kg) as I have no desire to do cat A, especially since there are normally very few guys on the start line for these (at least for EVR which has separate starting points for each cat) compared to B and C. Maybe if you want more of a race and less of a TT that is an approach worth considering.