a good quality diff in
But is it? Its a locking diff which as I understand it means there has to be substantially different levels of traction on the two wheels before it has any improvement over an open diff. Furthermore the stock GTI has the ability to break any wheel experiencing a loss of traction, which although not the same as a mechanical system will still deliver much of the benefits. So, one could argue this is a rarely needed feature where the stock option already delivers much of the benefit.
Since I'm playing devils advocate I'll also argue that the 'huge' difference you describe would need to be tested in a blind study. Otherwise, sure if someone gets in a car with better features I'm sure they will start to 'notice' the difference. But I really would be surprised if more than 1% of people could tell if they were driving a GTI with PP or without if they weren't told before hand which it was. I kinda wonder, for someone looking for a private purchase in 3-4 years, if the seller tells them its got PP and DCC, other than looking for "GTI" stencilled on the callipers how would they know? Thats near £1800 that may or may not be there!
On resell value, I am inclined to agree with you more. To be honest if I was going to spec a car to have maximum resell I'd put manual, PP and DCC, tartan seats, 3dr and dynaudio. However, I don't want to resell it I want to enjoy it, so I didn't go for any of those options!
EDIT:
Not that someone who did go for those options wouldn't enjoy it, I just mean I wanted other things and my budget was not limitless
