Whilst its not a jeep and its not that rack, the rack i do have on my landrover is similar, Have had 6-12' random lengths of soaking wet wood of the beech the full width across stacked 6" high up there, load wise its no issue, but cornering wise much mass up there has a big effect.
My rack although different has a similar number of bars and spacing, hence i can say with certainty a few sheets of ply it works great for, only thing to watch is how you tie stuff down, for things like stacked sheet goods you need a strap over the front and back edges to stop the middle ones sliding out. other than that things travel great up there. Other thing to watch is load - unload, any wind and its dodgy and heavy larger sheets are very hard single handed. I often will also have 9' concrete fence posts up there and there way easier than a full sheet of ply to load - unload.
Use to have a extension piece that slotted in the front bumper, but honestly it was really only useful for 4+ meter lengths of floppy stuff like smaller steel flats. Old bumper sarcame to rust and i just still have not needed to add the mounts back for that to the new bumper.
