I don't think anybody who has a GTD (including myself) truly believes that in a straight line, driven by the same driver that the GTD will beat the GTI, but at the same time there wouldn't be that big a gap as some GTI owners would have you believe, there really is f**k all in it.
Some of the comments on here from GTI owners are equally as daft as those who say the GTD is quicker. I've read stuff like 'Cos they couldn't afford the GTI' when there's only 500 quid in it and other such stupidities.
They're both excellent cars in their own right. The GTD suited me more because the company car tax and fuel costs were lower so I could spend more on important stuff like family holidays etc - much more important to me than how much faster my car goes than its diesel equivalent.