I think it depends where you bolt the cage, and how you crash. - In theory, yes the cage will protect you in a crash.
Where it gets grey is if the car was designed to crumple to absorb some of the energy of the impact. Make the car solid, and suddenly all the energy goes through you. - Probably not a huge concern on a twenty-something year old car; side intrusion bars in the door were considered an innovation then.
It's why big 4x4s are not necessarily the best car to be in during a crash; there's lots of metal around you, but it's designed to keep running when you hit something off-road and thus doesn't give.