That is also why you might not want to join a union. If you can show you are better than someone else (or at least think you can show), the union is likely to force you to get the same pay anyway. Unions often have promotion rules that harm anyone seeking to move up. Of course if you are content this doesn't matter.

Which is why unions work well for assembly line workers: the line moves at the speed of the slowest person on the line, so there is no way to be better than average. (the company/union can fire someone enough slower than average)

You might in fact be as incredible as you think you are.

Even then, the lift from collectively bargaining probably outweighs your individual competitive abilities.

Also, the union doesn't have "promotion rules." The members themselves at those unions decided that's what they wanted to negotiate with their employer into a contract. There's no union rule that says that's what members on the bargaining team have to prioritize. The bargaining team can even prioritize performance pay! I will, however, grant that the very nature of democracy (in this case, workplace democracy) means some people will disagree about what to prioritize.