Didn't you just say it was the individual not the system?
Yes, I wasn't talking about the system itself, the manner of strikes, grappling etc., I was talking about how the instructor helps make the "warrior's mind" so to speak. It doesn't matter what Martial art you use, if you don't have the right mind set and an ability to control and/or cope with the effects of the hormonal cascade that comes from fight or flight, the art is irrelevant. One of the ways to do this involves training BUT it is not a training specific to any art.
I'll use an example a method of fire arms training. Instead of just going up and putting holes in paper; sprint for 25 yards to the 15 yard line, drop and do 15-20 push-ups as fast as possible. Stand up and wait to draw and fire until the range officer yells "threat!!!" From the call of threat you have 3 seconds to put 2 rounds on target. Now that actual skill in firearms (martial art) doesn't start until after you hear "threat"...everything else is just an exercise to get your body under stress. The first time you do this exercise, if you have no other stress training, I can almost guarantee that you will not hit the kill zone and likely not get both rounds in the silhouette.
The same, in my experience applies to the Martial Arts. Many, if not all, of the tools to help teach the student to deal with this physiological dynamic though can be used across all Martial Arts, regardless of the mechanics of them.
You can do some training wearing a Halloween mask to simulate tunnel vision (which the exercise I noted above can't create as the tunnel vision comes from a HUGE adrenaline dump brought about by legitimate fight/flight). This teaches you to use scanning techniques. While Martial Arts have done this for centuries the modern military community actually speaks to the use ofhttp://loadoutroom.com/2778/tactical-breathing/ to help moderate the impact the stress and adrenaline dump have on your heart beat and this then Moderates the degree of tunnel vision and loss of fine motor skills. While learned these are can be learned independent of any specific art and they have a measurable impact on the individual.
As for the idea of proving that it is the art and not the fighter, you are essentially asking me to prove a negative, to an extent. There is no metric on which to reasonably compare due to all of the variables that go into any fight. The combination of skill along with relative size and strength, the way the encounter starts (sudden attack or on "the bell"), confined space or open space, actual experience not simply in the Martial Art, or even competition but in "real" (for lack of a better term) fighting, there are a multitude of variables.
The only way to come up with an objective metric would be to run a controlled experiment. To do this you would need identical twins, have them go through the same physical training, get the same experience in their respective martial arts, throw them into the same incidents so they share the same experience in "real" fights etc. This kind of "control" is impossible.
So all we can do is gather the data we can. We have accounts of every Martial Art, including MMA, having its practitioners lose in street fights against other styles. MMA guys have been knocked out by boxers, as an example.
It's not only that though. As a simple matter of logic, if someone claims to have built a better mouse trap, it is that person's obligation to prove it is objectively better. See above however because just as I admit it's impossible to prove my theory to a 100% degree of scientific certainty, the guy who says he has the better mouse trap can't prove that either because creating the control is impossible.
. Add to that those people that simply have a natural talent.
