Safety progress in S2000 compared to WRC?

[Daniel]

Mass affects car's kinetic energy, making heavier car more dangerous to those outside it.

For the crew safety, mass of the car has no direct effect. In the simple scenarion, mass of the car does not change during deceleration and it is shedding only the velocity component of car's kinetic energy. In broader sense however, heavier car can be made safer by sturdier safety features. Of course, this depends on rules, for example the increased minimum weight for the body shell was a step in right direction in this sense.

Also, heavier car has slower acceleration (though it conversely has slower deceleration), indirectly affecting passive safety.

Agreed. It can work both ways. Lighter cars are easies to stop etc etc but a heavier car may for instance break down a smaller tree rather than wrapping itself around it

[Mirek]

Mass affects car's kinetic energy, making heavier car more dangerous to those outside it.

Not exactly. If You hit anything solid You can't break by Your car (as Daniel says), than more weight with same rollcage is definitely dissadvantage because in the same impact You need to absorb more energy. You can do that only by bigger deformation or by bigger rebound or both. In any case it's always worse.

Simply You have car 1 with 1000 kg and car 2 with 1230 kg. Both are going 100 km/h and crashing straight into a wall or a big tree the way they can rebound only in opposite direction of original movement (no spinning around, rolling etc.). Both have same rollcage in this example because there is no rulle which says the heavier car must have stiffer rollcage existing.

Car 1 has kinetic energy 386 MJ.
Car 2 has kinetic energy 475 MJ.

The difference says how much more the second car will suffer from the crash.

For side tree impact there is also reasonable importance of car lenght. While in frontal crash more lenght is usualy better (more possible deformation), in side impact into the tree it may be often opposite. The longer the car, the bigger moment of innertia of the body which bents the car over the tree.