Got one 6 months into ownership back in 2014 after being underwhelmed by the MK7 GTD's apparent performance (felt a lot slower than the Scirocco 170TDI it replaced due to very linear acceleration). I had previously tried out a TDI-Tuning box which was horrible in fuelling and power delivery. I took another look at boxes, and tried DTUK, they were localish, to try before I bought - I was sold! It transformed the car, marginally quicker than a 230ps GTI PP in gear (although not off the line, due to tramping even more so than stock), tons of in gear torque. It pulled in 6th like stock does in 4th. DPF regen frequency halved, and mpg was not noticeably affected at all (when not pushing on, mpg is exactly as it is when stock).
Ran it about 18 months (including testing out the newer version) and then was thoroughly annoyed with my Bridgestone shod GTD's tendency to tramp and got an R.
The box passed to my Dad and he has thoroughly enjoyed it for 88k miles. About a month ago, my Dad's GTD has started having issues, going into limp mode and lots of dash lights. Those issues disappear with the box off. Not sure whether recent VW covert updates when in for servicing mean that the ECU is no longer fooled by its presence, or whether there is an underlying issue that the box aggravates (e.g. DPF significantly filling up with incombustible ash, clogging up far too easily), or perhaps even that the software/firmware of the box is corrupt and needs a reflash.
My Dad really misses the box, so next time he's up, he'll be after a reflash, just to be sure there's no way back to using it again without issue. If I had a GTD and I could be sure that VW wouldn't know I had one to put my warranty at risk, i'd have another one in a shot. No clutch worries, unlike the GTI and R, VW seem to have some margin on the GTD's clutch to handle the extra torque.
