Mitsubishi MN dual-cab ute spring concerns

We took delivery of five Mitsubishi MN series dual-cab utes with dealer fitted trays in January this year. When fully loaded all are under the recommended axle and GVM as stated in the Mitsubishi handbook. The weight is carried forward of the rear axle against the front of the tray. The heaviest grosses 1080 kg over the front axle, 1480 kg over the rear axle, and Mitsubishi's recommended maximum GVW is 2900 kg. The rear springs have sagged so badly that they have only 30 mm clearance to the axle bump stop. We are very concerned that our employees could be at risk when driving these vehicles. The dealer we reported it to bushed it off, but we have been told by a suspension expert this is a common problem with this model, and that they would reset the springs and add extra leaves at a cost of $500 per vehicle. The concern we have in doing that is that we would be altering the vehicle away from the manufacturer's specifications and might void the warranty. We have a mix of Toyota, Mazda and Mitsubishi utes, nine in total, all the same configuration and doing the same work, but the Mitsubishi is the only one to have this problem. What obligation does Mitsubishi have to repair them? What should we do from here to have the problem resolved?