Now that we think of it, this is the kind of comparison we need more of.
This is not the kind of race that we see a lot of, but now that we have, we wish we did. The idea is really pretty simple: take two examples of the McLaren MP4-12C, both of them completely stock, and see which one is faster. One of them is a 2012 model, and the other is a 2013. We know that improvements were made for the 2013, but how much of a difference did these make, if at all? Well, this is really the best way to find out.
It will still be up to you to determine if this is a definitive test, as the human element can always skew results. But that does also mean that no test can ever be perfect.