Let me ask the question in a different way to see if I understand it correctly.
Suppose the fifth wheel had a measured pin weight of 3k pounds. If you were to remove 1500 pounds from the pin, and install a 1500 pound hitch in the truck, would it handle the same.
I know that a trailer that does not have enough hitch weight (tongue or fifth wheel pin) will tend to sway more. In extreme instances, the tail wags the dog and can be very dangerous. I have always assumed this happens because there is not enough weight on the back of the truck. This results in less weight on the rear tires, reducing traction as well as moving the center of mass of the truck forward making it easier for the trailer to push the rear of the truck sideways.
However, in the above scenario, the truck might feel the center of mass and tire loading the as the same. If that is true, I might need to rethink my understanding of trailer loading.
With all the issues of frame flex and cracking, it would make sense to take the weight off the trailer and put it directly on the truck in the same way if it means the handling is the same.
This is an excellent question. My gut says if you can load the truck the same way, the trailer will handle the same. But I have no idea how to test it.