I am very concerned about the springs in my 2014 Mazda BT-50 ute, as they appear to be inverted, even before adding weight to the vehicle. I purchased the BT-50 to tow a bogie-wheel caravan with GTM of 2600kg. I have complained to the dealership who have tried to discuss the problem with Mazda, but Mazda's answer was that the vehicles springs are designed for a luxurious ride. A Mazda dealership employee told me that it is ok to ride around with the springs up to its stoppers. We are not driving around in Japan, we are "grey nomads" driving around in Australian conditions and this vehicle was supposedly designed as a "work horse" capable of pulling 3.5-tonne and have a capacity of carrying 1100kg. A number of other owners I have spoken to have added an extra leaf or airbags to alleviate the problem. Why should owners have to do this to enable the vehicle to carry/pull its capacity.
We have had one or two other complaints about the rear springs on the Ranger, which presumably is the same as the BT-50. Like you I would be concerned if the springs on my vehicle were inverted, and I would not be happy about them riding on the bump stops, and I too would question the vehicle's ability to do what it is claimed to be capable of. I would take it to a suspension specialist and have it uprated to do what you want to do with it.