Stiffer springs on the .2 RS as has been stated, but also softer anti roll bars - very like a cup car set up (on the .2). The only numbers I have on hand are for the front anti roll bar are that it has been reduced in stiffness by (or to) 40N/mm. The .2 RS is very close to a track set up and with the softer ARB balances out pitch, roll and yaw beautifully. Quite probably as close to as good as it gets for an out of the box package that can balance road and track duty.

For reference the front spring rates on a 991 Cup are 240N/mm (and 45N/mm for the helper) relative to 100N/mm for the .2 GT3 RS and 45N/mm for the .1 GT3 RS. The rear spring rates are 260N/mm (and 80 Nm/mm for the helper) relative to 160N/mm for the ,2 GT3 RS and 120 NM/mm for the .1 GT3 RS. In the 991 Cup, the front and rear anti roll bars can be varied in stiffness (depending on blade position) from 15Nm/mm to 58 N/mm.

Its worth keeping in mind its the balancing out of the three vectors pitch, roll and yaw that maintains maximum tire patch contact - the addition of helper springs all round also adds to this. There will be other differences too e.g. "spring" angle which may come into play - however I would Imagine on the .1 and .2 RS these are similar and possibly identical - but likely different to the GT3.

Unit conversions 100 N/mm ~ 10kgf/mm ~ 571 lb/inch and 160 N/mm ~ 16kgf/mm ~ 913lb/inch

 

Yes, cup car is a lot stiffer, especially in the front. But the racecars also use very different rubber.

Btw. the roll bars are also adjustable in the street cars. Interestingly, my race shop tend to set the roll bars in the front even stiffer than stock setup, even if a aftermarked suspension with stiff springs is installed.

.1 and .2 GT3 is simply to soft for the track and for my buttfeel ;D