I have had my standard manual GTI from new since July 2005, and I started to feel recently it had lost some of it's sparkle (maybe from over familiarity) so I got a remap from a local specialist. All seemed well and there was a decent increase in performance. A week later for piece of mind I changed my DV first to a Forge one then to VW's own revision D version.
After I had the Forge valve installed the car transformed into a bit of a torque monster, from 2000rpm you could feel the front wheels scrabbling for traction even in the dry and I started to occasionally get over boost problems. As a result thinking maybe the Forge DV was to blame I changed to the D revision. The problem didn't go away and then I also started getting slight clutch slip at about 3000rpm.
I then had the map modified to reduce the boost pressure and finally the problems went away.
Finally after driving about for a month I decided return to stock as I wasn't enjoying driving my car any more, I felt like something was going to break/split or explode at any moment. Don't get me wrong the performance was superb I estimate knocking at least 3 seconds 0 to 100 mph, but the fierce delivery from 2000rpm wasn't to my liking.
In my opinion if you are considering a remap on a (older) manual standard GTI I would consider uprating the clutch as well as the DV* or drive in such a way to protect the drive train.
I am not trying to put people off remapping it just wasn't for me, but I think it interesting to have some other views on the subject and remember this is manual not DSG which may well be a better candidate for remapping.
SJS

*I did wonder if my original rubber DV although not split was the reason in the first place that I thought the car had lost some performance, it was quite noisy and the revision D is virtually silent.