i may have some interesting input on this.
the car i have here for tuning of the stage 2 SC has a more "track oriented" shock and strut setup, with modest springs. they are the bilsteins. the owner does not really drive the car on the street, due to the feel on the road. he hates it almost as much as i do. he does track it. after a short ride in my car the other day, which i am pretty sure has stiffer springs than his car, he wants to change to the shocks and struts i have on my car. assuming the lengths are right, the springs will remain unchanged. he'll give up nothing in cornering and gain quite a bit in handling, just by getting rid of the non-adjustable gas bilsteins. the jarring and twitching will be done.
don't get me wrong. the bilsteins are capable shocks on the track. they just lack the civility this car demands on the road. if you plan to dual purpose a car, i would not use them without the expectation that some of the things that drew you to the 968 may very well be gone.
the reason for this is the nature of gas and the angular design of our rear suspension. the fronts are not so much of a problem, and gas is not so bad there.
the rear suspension puts the shocks at a steep angle. this means that for the same amount of ride height change, the shock must move more. it's about 40% more. that means that for every bump the shock is trying to absorb, the shaft must travel 40% more than in a vertical orientation (like the struts are). gas shocks have a preload charge, generally about 25lbs. when you rapidly compress that gas, it gets very firm very fast. if the springs are not matched to that rate, the result is jarring and twitching. gas shocks have a fairly narrow window of spring rates for which they are tuned. for a gas shock to work well, you have to be near the upper limit in spring rate. based on how they feel, for the bilsteins, i tend to think that they probably need around 400lbs wheel rate (.56 * coil spring. if you are using the torsion bars it is .56 * coil spring +175). that means you have a REALLY high spring rate for the car. that would make for a really rough ride on the road. reindexing does not help either.
with a hydraulic shock, the motion is more fluid (pardon the pun). the nature of the fluid means that it is harder to push through the valve, and thereby is more progressive in how it absorbs the bumps. this is what makes them superior for road use. while they tend to have a wider window of springs rates for which they can be tuned, they do tend to be more limited in maximum spring rate, as they can be too slow if the springs are too stiff. for example, the maximum spring i can use with my shocks is 600lbs, which results in only 336lbs wheel rate. i found that to be too stiff, and run 500lb springs, which is almost 300lb wheel rate.
due to the power to weight ratio, i tend to think that a smoother but softer car is going to be faster than a stiff one. this is due to weight transfer. a stiff car does not transfer weight as well, and consequently requires more power to push it through a corner than a car that transfers better. if you have limited power, that means you are at a disadvantage. i learned this with the mgb. while the car could hold a corner better with a super stiff suspension, it was actually slower in lap times, because it could not carry the weight through the corner, due to the lack of power. momentum was everything on those cars, and with our power to weight ratio, it is very much the same. i see the mistake of trying to make the 968 do things like a 944 happen a lot. the easily 400lb difference messes that up though.
it's no simple answer, and you have to decide for yourself where you are willing to compromise.
