All this physics discussion was driving me nuts, and the video linked only tells part of the story. It's a much more complicated situation than the video investigates, and even using basic physics (i.e., a linear system) you can show that the hitting a wall at speed V and two cars colliding at speed V is a very special case.
Body 1 with mass m1, stiffness k1, and speed v1
Body 2 with mass m2, stiffness k2, and speed v2
You can use conservation of momentum to calculate the speed of the 2 bodies after impact and once "stuck together" (m1*v1+m2*v2=(m1+m2)*vc).
From there, use conservation of energy where the original kinetic energy (m1*v1/2+m2*v2/2) is equal to the sum of the kinetic energy of the system ((m1+m2)*vc^2/2 where vc is calculated above) and the potential energy of the springs (k1*x1^2/2+k2*x2/2)=keq*x^2 where keq=k1*k2/(k1+k2). From there, solve for x and than compute the force F=keq*x.
So what do you get?
Let's suppose you have a car with m2=10, k2=20, and v2=-10 (moving right to left in my system) hitting an infinitely stiff and infinitely massive wall with m1=10000, k1=10000, and v1=0. You get F=141. If you just solve the single degree of freedom system the closed form solution is sqrt(k*m)*v, and the values agree (slightly off since we aren't quite at infinity in the original).
Now let m2=m1, k2=k1, v1=-v2, or two similar cars colliding at the same speed. You get 141.42, which is exactly sqrt(k*m)*v.
OK, so it works for the same cars colliding. But what about a slightly bigger car but just as stiff? Let's take m1=1.5*m2. This gives F=155, or about 10% higher contact forces. If the first car is also stiffer (say k1=1.5*k2), you get higher forces (F=170).
What if that first car is heavier (say m1=1.5*m2) but softer (say k1=.5*k2)? Then you get F=126.
How about hitting a parked car that is identical? Then you get F=71. Make that car infinitely massive without changing the stiffness and you get F=100.
In a real system, the masses of the cars will be relatively similar, unless of course you are talking about a Bentley hitting a Elise, or 18-wheeler hitting anything! But in your standard race with similar class and weight of cars, the masses should be similar. However, the big unknown is the stiffness, which depends on the materials used and the design. Collisions are also highly nonlinear, and nonlinear stiffness effects come into play as does nonlinear material behavior (plastic deformation).
So is hitting a wall at 50 mph the same as two cars each going 50 mph colliding? The answer is yes, but for a very specific case (infinitely stiff wall and 2 identical cars). In reality, it's not the case, as simple physics demonstrates.
That said, the damage system in GT5 is garbage...