Including some of the aircon system and adding back in 2 bucket seats and harnesses I have removed about 100 to 110 kgs.
Depends on how drastic your planning to be. The above figure includes door cards, headlining, swapping electric windows for manual ones, stereo, speakers etc etc.
I have got a rough breakdown of the weights if you need it.
This was on a 3 door 16v GTI
Paul