I understand there may be some debate about the actual purpose of breasts, which is why I phrased this as a hypothetical

What you did was make the following rather direct claim:

The obvious breaking point would be that breast implants, at least, are improving the trait directly rather than improving signaling of said trait.

[...]

Of course there are differences between the two, and further differences between breast implants and makeup but the 'breaking point' most certainly isn't clear!

I guess I did phrase that too strongly, but adaptation-executors, not fitness-maximizers.

the ability to perform social acts that combine dominance, humor, rapport and charm can be declared to be "an actual trait that is desired" as well as "a signal about other traits".

Well, yes. As I said here, some traits may be (un)desirable in themselves as well as signalling other (un)desirable traits. The benefit of your signal could outweigh the harm of what you're countersignalling. My point stands.