Hi,
I had to load 400 pounds of cement bags to get the suspension to sag 2 inches. I have Elka stage 1 shocks though. I've also had more than that with mulch in the back, and it didn't sag any more than that. I've also did an experiment with water softener salt.
I know that doesn't sound like much, but you have to remember that the suspension travel is only 5.9". I believe a proper suspension sag setup should be roughly 20-30%. Let's use some numbers that are pretty easy to work with.
500/520 suspension travel is roughly 6"
25% of 6 is 1.5"
Sag of weight from a full load is another 2"
That leaves you roughly 2.5" of upward travel.
I actually think I have a thread on my suspension tuning from last year that might be a good read for you.
Rob