Milltek - Downpipe definitely will fit, cat back system won't.
ITG - as said you just need the hose that fits on to the turbo as it has a take off pipe for the DV return pipe.
Twintercooler - This is a funny one. You definitely need different hoses for a start and the actual twintercooler core may have different brackets on it. If I had a GTI one in stock at the moment I'd compare them for you but I don't just have the S3 kit. For the price of the hoses and messing with moving the brackets and what you could sell the GTI kit for on here I'd say you are better off selling the twintercooler and buying a new S3 one.
In the end I don't think its going to cost you that much to swap it all over