I have the Ironman FCP kit on my GX470 and use my GX in a similar way as yours (DD with off-roading and towing). I went with the lowest load setting for the front and rear coils and it is stiffer than stock and squats very little when loaded up with a few hundred pounds of camping gear. Unless you are also adding front/rear bumpers, fridge, RT tent, etc, I'd stick with the +0-110 lb rate springs. Or, as mentioned above, get the +0-110 lb rate springs and throw in a set of Firestone bags if you still get sagging when you load up with camping gear and bikes.
Regarding overall Ironman quality, the FCP shocks are super-beefy (2.8" shock body), rebuildable at home, and I have been very happy with how the setup handles on the highways, rough dirt roads (where it really, really shines compared to stock suspension) and off-road (more articulation). It was an great upgrade compared to stock and the GX is way more controlled and less floaty in every situation, although the ride is a bit stiffer (which I personally like). The fit/finish/quality is very good and the FCP shocks have been well-tested by the 4Runner and Land Cruiser folks for quite some time.