I am not a fan of the forge twin inter cooler setup there is still going to be a pressure drop across the stock core and cannot see the reason for doing it, gimmicks??. The S3 cores bring the pressure drop down by 3-4psi and imo one of the best oem mods for the gti for both cost and performance. The THS is a good setup and works well but only going to improve over the s3 cooler if the setup is running 340+bhp. Only splash out money if its needed, stock with the cheapest option for your target power which in the case of a k03 tfsi is the S3 290quid oem core which will happy flow as much as the k03 can give.
Nick