I can't imagine why longer shackles would make the leaves sag since they are not bearing any addidional weight & for normal street duty, the springs wouldn't flex any more.
In bush bash mode, the greater clearance that the axle has to the bump stop might allow a greater degree of flex & perhaps it's this that results in sag, but it wouldn't be the shackles per se'

If bottoming out is the problem then long shackles only address the symptom, not the actual problem & the fix is very likely going to involve a chat with a suspension specialist & some re-arched springs, possibly with a stronger spring rate if bush bashing is what the car is used for. Perhaps some good shocks too.