As James said... that is a good question ...and one that can have many philosophical interpretations... I believe that there are many degrees and definitions that can be applied to the words "restored" and "restoration". I think that is borne out by the fact that MCA has different judging classifications for people to compete in, depending on the degree to which the car has been "restored" and how it is used..i.e. trailered vs driven.
For the sake of discussion let's take a look at your control arm example, which by coincidence applies to my car. In your example you rebuild an original control arm. In this case I would assume that you have probably replaced the ball joints, duplicated the rivets, rebuilt the cross shafts,then repainted. In my case I purchased NOS Ford arms in 1978 from my local Ford dealer and finally installed them in 2010. I would agree that from an originality point of view your example is more original than mine, but is my car any less "restored" than yours or more "restored" than the guy that bought a new Moog unit?
If we are strictly talking about judging, then I would concede that you have a more "original" car than I, but from the perspective of using the word "restored" to describe both cars I think we both are correct.
When looking at various restorations it is very clear that some cars are done to a very high level of "correctness". The great job done on the K convertible is a prime example. If I were to compare my car against it...I would quickly say my vehicle couldn't hold a candle to it from a strictly original point of view. However, in my project as in the case of the convertible, not a single nut or bolt was left untouched. Everything in my car is either,new or professionally rebuilt and I tried to remain faithful to what Ford did when the car was built.. From that perspective I would call my car "restored" albeit not to the same degree as the convertible.
I know that we won't answer this question to anyone's real satisfaction here.. because each of us has our own perspective on what constitutes "restoration" but it is fun to have these types of discussions....gives fuel for thought and as long as no one is offended in the process I think we can all learn something new.
Just some thoughts....