I had my tensioner changed to the latest one last summer when it was four years old. It was sounding increasingly ropey on start up at that point so thought I'd go for a preventative change. It was one of the first ones as I got it in July 09 and was up to about 40k miles by last year. The guy who fitted it said there was a bit of wear on it but it wasn't actually near to failing.
Touching lots of wood as I say this it's been fine ever since. As I understand it the tensioner and the inlet manifold seals are the two obvious things that go on the Mk6 GTi so I sorted the former and the latter failed in the last year, fixed after goodwill from the dealer for £300 odd. I treated the tensioner change as if it were a cambelt change which you often do at 4 years so if it helps keep it reliable then it's worth paying.
As said before, I'd negotiate the fitting of a new version (or at least the cost of it) in the purchase price then not worry about it. Other than the above mine's been totally reliable for over 5 years now.
