Some success with finding a solution to the rotor offset issue:
Brembo rotor for Vauxhall/Opel
This rotor is 280x25mm with the offset to move the rotor friction surface 8mm outboard compared to the original rotor, e.g perfect for our application.
Now for the bad news, they slightly too large. Could probably get away with 277mm but 280 is just too big. They will need to be skimmed down somewhat to fit.
Secondly although they are 4x100, the centre bore is miles off at 60mm vs 54 for early GT's and 55mm for everything else. Fortunately this can be remedied with a machined hub centric ring. At some stage I will attempt to hunt down an affordable supplier of these in NZ, however as they're fitted to euro only cars I might not have much luck! If somebody over there could pick up this torch we could confirm if this will resolve the offset issues and avoid the need for a weighty and dangerous spacer or difficult machining of mounting surfaces.