Mike, I just read your previous thread of when you need a cage. Agreed, 4' off the ground at 100mph. Your question now is about covering the cage for a mostly street car. As someone pointed out earlier these cars are small. When you start adding a cage it starts getting tight.
Look at a cage as part of a safety
system. You want to be secure in a seat. May mean going to race seats. Race seats are designed to work with 5 point harnesses. Race seats and harnesses are a first class PIA on the street.
Keep in mind that all seat belt/harness material is made to stretch to help absorb some of the energy of your body coming to a sudden stop. (I seem to remember 15% as a minimum on new belts, older belts are more.)
Brad said you only need 2 pieces of padding. I'm not sure that's true, depending on the group you're running with.
I think a good rule of thumb is to pad any bar you can touch when securely belted into your seat. You're using the right stuff, the hard foam padding.
To give you some idea of the energy your body has to deal with in a crash look at
http://www.isaacdirect.com/ When I built my car I ran the same door bar as you have for ax. Last month I had it cut out and put in NASCAR door bars on both sides. I have gutted doors so I was able to have the bars go out into that space. Decided I wanted a little more protection 4' off the ground at 100mph.