I've been reading a lot of this stuff (crap!!) about GTI against GTD. I part ex'd my Ed30 against a second hand GTD (would never buy new - crazy prices) approx 6 weeks ago. The change was right for me because I needed to slow down(!!) and have a car which is better for long journeys (in-laws live in France and I'm in Scotland).
Anyway, to the point. A GTD is NOT a GTI or even close, neither in outright performance or in handling. The very nature of the diesel dictates that it can't match a GTI in initial acceleration. Yes, once on the move it can accelerate quite rapidly, but by then, if from a standing start the GTI will be long gone. A MK5 GTI left me for dead last night and while initially I tried to keep up, I soon realised it was pointless, plus I was leaving such a large amount of black smoke in my wake, somebody probably called out the Fire Brigade!!
Handling wise, it is also lacking. The extra weight up front means it will understeer a lot earlier than the GTI and I know this just from the way I could attack roundabouts and corners in the Ed, which I can't do the same with the GTD. Even a mapped GTD may be quicker, but it will have the same problems in the corners.
Don't think I am slagging the GTD off as a crap car, because it isn't. The inside is a much nicer place to be than the Ed30. It will easily do over 50mpg and travel approx 200 miles further on a tank than the Ed could manage. Cheaper tax, insurance etc.
However, those of you that have bought a GTD and are trying to defend it as a GTI equivalent are kidding yourselves. It is not and never will be a GTI - fact. Be happy with your choice, you have bought a great car, you don't need to defend your decision if it was right for you.