I've known Andrew (via another forum) and known of his company (and it's customer service) for a good few years now and I would be pretty sure that anything he sells via his business has been thoroughly tested before going to market. I very much doubt that there is anything on these boxes for the GTI that will cause any problems - so much so, I hope to get one myself as and when I get a Mk7 GTI!!
Genuinely don't worry folks, DTUK have been doing this for years and all I've ever heard is good things.
No connection with Andrew or DTUK apart from the above, promise!!