I've never built a ROPS, but have built cages, roll bars, and tube-bodied race car chassis for Baja racing and similar events. Some of them have rolled; no driver or passenger has been injured.
The critical things, in my experience, to ensure when preventing injury are basically to keep the operator within the envelope of the structural members (ie, seatbelts) and to keep the surrounding structure intact, by separating attachment points, having additional bracing, and gusseting junctions when possible.
I'm frankly surprised at the ROPS offered for protection most times on this size tractor. It has always seemed to me that they're barely going to be sufficient, if they don't collapse. That's why I would want to triangulate things as much as possible where it joins to the tractor. Or, make a 4 post style cage, which would have the added benefit of allowing a sun canopy to be attached. My reference is always to my desert race car background, though, where we build cars to crash at speeds orders of magnitude faster than a tractor can even go. A slow tip over will be less violent, obviously, than a 60 mph endo.
There multiple ways to do it, and clearly the factory style, two point ROPS are judged to be sufficient. That is definitely better than nothing. I'd just prefer more.
