not positive, I'm sure others will know more than i, but I think that the message Rover was trying to send was that their trucks were tough enough to do the job right off the showroom floor.. It was my understanding that the trophy trucks were all stock except for the added accessories, i.e. winch, roof racks, etc.... why would they use hd suspension parts while using those "bicycle" tires/wheels that they all had!?! maybe heavier springs for the extra weight?
I know LR does in fact sell heavy-duty OEM springs. They're available over at BP Utah, front axle only. I'm not sure how (or if) that translates to the G4 suspension, but I know that all G4s came with the rear air suspension which doesn't use coil springs on the back axle, leaving the fronts only.
Maybe they used the HD front springs to compensate for the additional load of accessories?
At any rate, they sell for $130/ea. You can get a all for Terrafirma Medium Duty springs for around the price of 2 of the OEMs.