If I'm allowed to vote, I vote for presenting just the defsetf because it's the form that allows you to write (setf (mycar ...) ...) in the first place. In your example, mycar is also a defined function but, for defsetf purposes, that's irrelevant. As a result, it shouldn't be presented as relevant. But that's just my opinion and I will not be bothered if you choose to implement it the way as you seem to prefer.
OTOH, it's the same question as whether presenting all the methods with the same name, or jumping to the one directly where you pressed M-.