You already have what I have... OMEs. Mine have not sagged noticeably and I've carried all sorts of things I shouldn't have, including at one point a Ford 8.8 that stayed in the back for a few months, another time a d30 that stayed in the back for a few months and was joined for a month or so by an AW-4, an NP231, and about 75lbs of tools. My carpet hated me but the springs just kept on going.
I recently added another 1.5" of lift in the back with a set of teraflex boomerang shackles, didn't mean/want to but I needed more length in the shackles to really get any flex out of the springs. I probably scragged the crap out of the springs for the first year or so I owned them because stock shackles didn't let them flex much, most of the suspension travel I got was the centers of the leafs inverting a bit. Still, they haven't moved height wise. They do flex a hell of a lot better with the shackles added though.