So I removed the bolts thru the bumper for the center hinges. That didn't change how far the ramp opens as shown by this pic.

All that happened is that instead of flexing the bumper, the hinge leaf now flexes and will likely bend the hinge rod if the ramp is pushed fully to the ground.

Anyway, this shows that the springs are too strong for the door weight and even if the bumper is reinforced, the ramp will still not drop to the ground. So I went on-line and looked at pics of identical units for sale. Shockingly, the vast majority have only two coil springs (including some at the dealer's other stores), although several had 4 and even 5 springs. Doesn't seem like a well controlled assembly process. But anyway I've got an appointment at the dealer and will ask them to try 3 or 2 springs AND reinforce the bumper.