Right...I've done more research, spoken to various people and here is my conclusion.

The GTD is 10mm lower than a standard golf. So you need to allow for this when lowering your car. If you make it very low you need to change the whole set up e.g either coil overs or springs and shocks. I don't want to do that as it would cost too much. I also don't want to go very low or hard. I'm no spring chicken.

There is 30mm between the top of the wheel and the arch on my car.
Eibach Sportline advertise 40/50mm drop. Minus the 10 and you are looking at 30/40mm. However, you need to change your shocks otherwise it will be a very hard ride.
Eibach Pro Kit advertise 30mm drop. Minus the 10 and you are looking at a 20mm drop. The stock shocks will be OK but might need replacing a bit earlier (50k). The ride should be good. However, you will still have about 10mm above the wheel.
So I'm going to experiment with the Pro Kit. I let you know how I do in a couple of weeks. I'm probably mad but there are a lot of positive comments on the US and AU sites about this setup.