I have a full turbo back Milltek system on my 6R, had it on for a year (about 10,000 miles now) and it's held up very well. However, I'm currently saving up to upgrade to a BCS system myself as they have better reviews generally.
Plus, anyone who looks at the Milltek warranty on the exhausts will realise that anything over a 30% power increase and they don't honour the warranty, even though they know full well a stage 2 tune exceeds 30% stock power on most occasions.