Oh hello there!
Long time lurker here, finally decided to create my own account, yay!
I have owned a 2005 AXX gti for a few months now, and with 170000km on the clock the diesel-idle was pretty much a given.
At first I didn't care about it too much but it started to develop a noticeable rattle when revving a bit, and the diesel sound started to rival most tractors so I decided to finally do something about it. I took a gamble and only got the cam chain and tensioner (and all the seals) replaced. The garage had to drill out the adjuster bolt but thankfully nothing of value was lost.
Just picked it up this morning and all i can say is wow... Sounds like somebody changed a totally different engine to my car. Nice and quiet, pretty much like any other petrol engine save for a bit of clicking sounds from the injectors and fuel pump.
As to your question of whether to get the expensive cam adjustor replaced or not, unfortunately there is no definitive answer. Based on what I've read here and what I've heard elsewhere (and my recent experience of course), the odds are quite good going without the adjuster.