So on the subject of R32 ARBs, as above, they are 23mm front and 19mm rear out the box, and with the 8L S3 sharing the same floor plan, they are a direct replacement, and 100% fit. But bear in mind the R32 ARBs are designed to cater for a heavy V6 up front, not to mention the standard fit Bilstein shocks and H&R springs out the factory.
I would recommend against this mod if your suspension is standard, at the very least, you should have fresh shocks with lowering springs, but uprated aftermarket shock is my preference, but that's just my opinion.
Based on my personal experience with the 19mm rear ARB alone, yes, the S3 handles a lot better, but only because it was so poor in the first place do we have this opinion. My experience of just running the rear ARB is a twitchy back end at high speed(anything over 80 on a private road). I also had a 'moment' of fish tailing, when I hit a bump in the road whilst doing an emergency brake from high speed when a car pulled out in front of me, but my rear tyres hung on and saved me. And of course my infamous sideway vid, the first track outing with the 19mm rear ARB fitted managed to get me side ways.
So what I trying to say here? I wouldn't recommend the 19mm rear ARB as a stand alone mod, its edgy if you explore the limits of your car occasionally, and if you run budget tyres, I would high lye against this mod.
So what did I do to combat my problem? What I wanted was a 22mm front ARB, I tried that setup before on another S3 I owned previously, which Gops now owns, but Neuspeed no longer makes this ARB, they are discontinued! But lucky by chance, I found out, well, another forum member did, that you can buy a 21mm front ARB direct from VW. It's still classed as an R32 part under quattro systems on ETKA I think?, but it's non Uk spec, so probs US spec? I don't really know. But it's been fitted now, and what a difference it made!
I've not actually said much about this until now, as I was testing it myself, but I can tell you it works for my suspension setup, and it works for my driving style. But this has all been done on OEM 17" wheels, need to try it on my 18" BBS rims before I'm totally happy.