Chassis ridigity is always a good discussion, but, it always raises a few questions.
1. What figure are you looking for.
2. How do you measure it.
For me the first is that you are looking for a stiffness a magnitude above the stiffness of the suspension. Effectively what you are looking for is the suspension to do the movement where you can control is with the shocks, and tweek it with the tyres, arb's etc.
F1 tubs aren't as stiff as touring car shell, primarily because a toruing car will have a massive advantage in terms of the chassis width so its sectional properties are higher. Stiffness to weight ratio obviously being in the F1 cars favour. You might have 15,000 Nm/degree in a carbon single seater tun and well over 20 in a toruing car. If you are running massively stiff suspension like these cars do then the very stiff shell is a big advantage and you can argue the extra weight to achieve it is worth it.
The second point about how to measure it is very interesting, as it makes a huge difference to the figures you get. In all my time in racing i've constantly debated with many other engineers the best way to do it. The answer seems to be that there is no ideal way to do it, but, you must do it the same to be able to compare. So any figures that you see written may vary greatly from what you would get testing it with your preferred method. You get this with FEA of parts too. The restraining and loading method alters the result so much that you can condemn a perfectly good item if you don't get the FEA right.
Ross Brawn is quite famous for not being overly worried about torsional stiffness, he prefers to analyse the weakest link in the chassis' stiffness and concentrate on that. He believes the chassis is only as stiff as this weakest point. I can kinda see what he is getting at.
Added this bit after reading some of the other thread.
The reason that the method of restraint matters is that the stiffness of an item is largely dependent on its sectional properties. The axis it twists about referencing the distance the extremities of the chassis are from this axis is what largely determines its stiffness. Imagine a very simple system like a driveshaft. You load one end about the centre of the shaft and restrain the other end about this same axis, so the cross section of the shaft is also located about the axis, all points being as far away from the axis (known as neutral axis ie point at which no twist occurs).
If you now think about how to measure a chassis, how you restrain it and load it determines where this neutral axis will be, and therefore how far away the structure of the chassis is from the neutral axis, thus the result you get will dramatically alter.
D.