In answer to your original question about improving impact survive-ability, the first thing you need is a better seat (a FIA approved seat) and an improved seat-belt (a four or five belt harness as opposed to the two belt harness you have now).
When you see those crash test videos of a car hitting a barrier and the air bags deploying, notice that the crash test dummy is flapping around like a rag doll. This is because the seat is flexing, and the seatbelts do not hold the driver in the seat. Now look at some in car video of a NASCAR or touring car crash. Notice how securely the driver is held in the seat, and while his arm may flail around his torso doesn't.
In a collision your stock seats will flex and move like jelly, having made your car stiffer (by installing a roll cage) won't help because your torso is free to flail about.
The seat-belt company schroth, makes a four belt harness with an inertia reel, (Schroth AutoControl II) which nicely bridges the gap between race only four point harnesses and typical street two belt systems.
Hope this helps.
A few points:
1) Any 4 point belt besides (arguably) the Shroth is dangerous due to submarining (the lap belt riding up and causing severe internal injuries by squeezing on the torso). Schroth claims to have resolved the issue with their "ASM" system, but I'm not willing to bet my life on it. I would only use a 4-point in an Autocross on an open course where it's really just to hold you in place, not protect in a serious crash against a hard object. Installing a 6 point is really not that much more difficult.
2) There's no reason to install a 5 point instead of a 6 point unless some obstruction in the car prevents installation of the 6th point. The 6 point is less likely to cause genital injuries in a wreck. Prices are about the same these days.
3) Harnesses have a problem with causing neck injuries if used without a HANS device (not practical on the street as you have to wear a helmet and have limited head movement). Your body is held in place, but your head isn't unless you are using the HANS. A 3-point with an airbag is going to be safer on the street than a 6 point since the airbag helps prevent the head from snapping forward. Since our Alfas don't have airbags, it's probably safer to be in a 6 point than a 3-point, but I don't think there's much hard data out there on the topic.
4) As stated above, a full cage can present head-injury risk. However, a FIA rated seat and harnesses should help prevent this. A lot is going to depend how far the halo portion of the cage is away from your head. That's going to depend on the build, the vehicle, the driver, and the seat setup.
5) Remember, that safety is a system. Everything has to work in concert, and adding only one item can hurt safety. Installing a harness without fixed-back rated seats, for example, may make things worse than no harness. Adding harnesses without roll-over protection is likewise dangerous, not to mention the fact that without a harness bar attached to the cage, it's unlikely you are going to get a proper mounting angle for the shoulder straps.