It depends how much you want to sacrifice ride comfort. There was a member here not too ling ago put 20's onto his astra g. Iv also seen a few astra h with 20's I had 19's on my cdxi vectra which was lowered with eibach springs and that was a pretty rough ride. Once you start going above 18'' on an astra g then you need to do some modifications to the arches and the plastic liner to stop scrubbing etc.