What would be the effect of having these items (including your other suggestions) on a car that did not have any additional stiffening beyond the factory torque boxes. Street driven, not competition in my case. Would there be any handling improvement? Would the lack of extra stiffening potentially cause the metal to buckle somewhere while driving?
Yes, there would be a handling improvement using that set up as it is a big step up on wheel rates from original and is in the upper range of what a lot of "pro-touring" type cars are running. However, you don't realize the full potential of the changes when the uni-body structure starts flexing. When your putting suspension laods that are this big of an increase, your body is going to be moving around. Similarly, once the body becomes an active participant in the suspension motion, handling can become unpredictable as you expose each new weak area. Check out the XV video on chassis movement and you'll be suprised how much these things move in their stock form. While it is unlikely that your car would just come apart, over time it is possible that you would begin to pop spot welds here and there and as each weld yields, you increase the load on remaining points. That could eventually lead to buckling or tearing an exterior panel. My guess would be the first place to start showing fatigue would be seams where the roof attaches.
Now, if you want to avoid the rollcage and still keep a stealth appearance while strengthening the car, you could stitch weld all the seams of all the sheet metal structures. An x brace under the car instead of conventional subframe connectors is also out if sight, out of mind. The XV under fender and core support pieces are also helpful and not easily noticeable. The area of the car most in need of support is going to be from about the front suspension points to around the end of the door area where the leafs springs mount. This area is subjected to the most torsional loading and any additional support you can provide here goes a long way to firming up the whole structure. Would it be enough to avoid damage, hmmm, maybe. It won't be as solid as a roll bar/cage set up, but it a lot better than stock.
Even if you aren't planning a step up in wheel rates, a solid foundation goes a long way towards improving every aspect of driving your car from smooth cruising and quick steering response, to eliminating squeaks and rattles. IMO all of our old cars should have sub frame connectors in them at a minimum.
