Hi there,
I have a full EFS setup up in my '06 STR. Love it! Really improved the ride.
HOWEVER..... I went for the medium rear springs. They are fantastic when there is nothing in the tray etc, but they are now stuffed after a Simpson Desert crossing.
I have been told that they were overloaded and aren't covered by waranty. I have my doubts about this but this is another issue.
My advice is that if you go with medium (150kg - GVM) you will need airbags as well for the times you actually use your ute as a ute. These springs abviously cannot handle a heavier load (anything over about 350kg).
I am going to get my springs reset (at my expense) and get airbags for mine, as I don't want the stiff ride of the heavy duty springs when I have no load. Hopefully this will keep them nice and flexy for offraod play time aswell.
Hope this gives you sme food for thought. As I said, I am still a big fan of the EFS gear! It has transformed my Nav. Just don't use the three year warranty as a major selling point.
Cheers
Timbo