JC,
The main safety function of the cage is to prevent collapse of the shell onto the occupants. Once that happens, you are in big trouble. The idea is to keep a protected space around you, and then use seats, harnesses, H&N, head side support, helmets, cage padding, etc., to take care of the rest. The recent FIA cage upgrades are to reinforce the basic cage structure to help it maintain it's shape in a wreck; e.g., the roof bracing and A-pillar supports are there to reinforce a heretofore weak spot: the upper A-pillar area.
Cage rules evolve, and the side impact work ongoing with FIA is a result of a death 1.5 years back in a side impact. This is certainly the hardest thing to achieve in a rally car since crush space is not there like in the front and the rear.
Build the cage to better than current standards, adding in some of the optional braces allowed. If you want better, then you need to look to an engineered cage; the specs are minimum standards. If you are building a car for which a homologated cage kit is available, you might look at that, but realize that sometimes these are built as much for light weight as for crach performance.
After the cage, you should put in as much of the head side supported seats as you can afford, and make sure the harnesses are properly installed. Beyond that, only some side impact cushioing can be added; this is a new area of research.
Finally, if you don't want to ever get hurt or killed in rally, then you have to abandon rally. No safety system will insure your injury-free participation in rally, just as you cannot be guaranteed to survive the next trip to the grocery store, whether in a car, train, or on foot. Racing has some inherent danger at all times, and rally is no exception. All you can do is minimize your odds of injury or death by implementing the best current knowledge of crash safety. THEN, always keep in mind that the one factor that you always control is how agrressively you drive. I can think of only one US rally competitor death where the speed or car control exercised by the driver was not a significant contributing factor.
Good luck and realize that the probabilities ARE very much in your favor to have a happy and injury free rally carrer. Regards,
Mark B.
(A student of race safety, who realizes that there is very little original thinking ever in race safety......)