Unions like to set salaries, because there are more mediocre performers who stand to gain compared to free-market wages than top performers who stand to lose. They don't exactly want a cap, but they greatly desire a floor, and accept it also serving as a cap as an acceptable tradeoff.

Whenever that first part isn't true for a union, then the market pressure is for the top performers to leave the union for free-market wages, until it is.

It was my impression, not supported by much, that unions do like to set a salary for a role rather than letting that be up to the company or negotiated between the company and the individual employee. (Unions featuring superstars, like the Screen Actors Guild, are an obvious exception.)

I think it's an interesting question how a government workers' union would be likely to feel about a hypothetical restructuring that lost 75% of existing jobs while doubling or tripling wages. I tend to suspect that if the union already existed and represented those low-skilled jobs, it would be adamantly against the restructuring.

Doubling wages while not cutting jobs would obviously be fine with the union, but what would the point of that be?

Rarely, but if the union official gets to chose between more people paying dues and fewer people paying dues, which are they going to pick?

We know that Unions don't oppose salary caps, but they oppose differentials in compensation based on job performance. Seems like that amounts to the same thing? Unless you're suggesting we just pay government workers more and see if we get any marginal productivity out of them.