There are a lot of variables to this question. If you are going over a rock, at a snails pace, and flip over slowly, it will hold. If you are going 70 mph, and loose control of your Jeep, and start flipping, not much will hold up to that. The aftermarket roll cages are definitely stronger than the OEM cage. I put in the Poison Spyder roll cage in my YJ, because I wanted the best protection I could get. But I also do more with it than I would in a regular Jeep. I would not try to climb a near vertical wall with the Rubicon, but I would with the YJ. So the chances of me flipping over back wards(the hardest landing there is), is very likely, if I am not careful. So in summary, it would all depend on what you want to do with your Jeep, and how radical you want to wheel. One more thing, I would have a lot farther to fall than the average Jeep as well.
I have to agree with Amerijeep, as there is no vertical support to your roll cage, on the OEM, at the windshield. If your windshield frame collapses, so does your side roll bar.