If it is like the 140/170 differences then it'll be different cams, injectors and turbo. On my Roc there was a 20kg difference between the 170TDI GT and the 140TDI GT (non-bluemotion), I presume most of that came from the mechanical differences as the only other difference (equipment-wise) was the addition of chrome exhaust tips on the 170 and they don't weigh much. With the massive difference in equipment between the 150GT and the 184 GTD, trying to determine the weight difference for mechanical differences would be pointless.
