^This. Regarding your question about STaSIS - that's going in the exact opposite direction from what you want. STaSIS (and many other after-market suspension companies) are in the business of selling systems that are intended to "improve" handling by stiffening the suspension (for less body roll) with stiffer springs and/or antisway bars, using shocks appropriately matched to the stiffer springs and ARB, and modestly lowering the center of gravity. One side effect of all this is that the stiffer suspension transmits more shock energy into the chassis, so if you're concerned about the suspension holding up under punishing road conditions this is not the way to go. Consider your rally car example - the distance of suspension travel (how much the springs and shock can defelect before bottoming out) on a rally car is much greater than the distance available on an S5, so the suspension can absorb the shocks with comparatively less force jolting the cabin. My suggestion is to follow the advice from patrick above - go with 18" rims for winter, or even 17" if you can find a set that fits (some 17" wheels won't clear the S5 brake calipers, but some can, and spacers may be required). The tires can then have higher sidewall to provide more cushioning from pothole impacts.