For what it's worth, I see the FWD S3 conversion as a pretty expensive and time consuming project.
You'll need to get a new box, and fit an LSD in it, which is going to cost in the region of £1000-1200. then there's fitting on top of that. Then you'll need to remove the prop, rear diff, rear shafts etc to get your weight saving. With the labour charge you'd be looking at £1500-1700 all in I'd bet.
What you'll end up with is a car that's more lively, and ultimately faster on the limit if driven at that limit. It won't be as good on the road though, as you'll always be battling wheelspin on road tyres, and again, on road tyres, it'll be fairly scary in the wet.
I'd definitely agree that a stock haldex car is not what you want for track work. In the dry they just seem to understeer like FWD with an open diff, and in the wet, the haldex becomes really unpredictable and engages mid bend taking you from understeer into oversteer without warning.
For your uses, and where you're at right now, i'd genuinely say that spending £500 on an HPA controller that you can customise will probably yield the best gains. you'll be able to set the car up how you want it in terms of 4wd balance. it's a really simple plug and play install, and in the wet you'll probably get on with it far better too.
That's not to say that FWD can't be fast in the wet. On full wets wet grip is incredible, and you'd happily run rings around a 4wd car on road tyres, but given all things equal, I think 4wd is still going to win in the wet, as there is still a limit to how much power you can deploy on a wet surface.
Lets look at it another way - You could get the performance haldex controller now for ~£500 or whatever they cost, and if you still didn't like it, you could probably shift it on for 80% of new cost anyway, so wouldn't lose that much at all.
