You will get more difference in geo set up than between a C2 and C4.
C4 can be converted to a C2 for £120 worth of CV joint hubs and around 2 hours labour.
Buy the car based on how it tight feels imho.
I bought my C4 with the intention of converting it to RWD, but the reality is I really, really like the extra weight up front, the steering and turn in feels better to me, like when you have just filled the tank right up on a C2.
So I have not bothered.
It is actually 35kg for the complete AWD system on the scales. The 54kg weight difference comes from all the other bits that the C4 came with as standard over the C2 including more material in the frame up front.
If you want it to feel more lively you can simply have zero toe in on the geo, or add a smidge of toe out if you want the back end to become playful.
I wouldn't even consider a tip in one of these, and I am a massive auto fan, bought my first auto in '98 with an E39 528i, being 24 at the time all my mates called me grandad, but I love autos, but not for me in a 911.
That changes the dynamics of the car far, far more than C2 vs C4.
When it is cold and damp, on PS4 tyres, the C4 definitely seems to fair bloody well, certainly more grip than I remember from previous cars, but that could be tyres or geo rather than going to AWD?
I have not seen the PSM light ever come on since refreshing the suspension, fitting PS4 all round and getting the geo done how I like it, no matter what the conditions.