I dont like to cause arguemnts, but i have to correct you here
Offset is relative to the hub, so it doesnt change the offset of the wheel. Your hubs might have moved, wich hasnt changed the offset of the wheel (how could it!). Instead its changed the offset that your car can run.
But i wouldnt have imagined that the hub would have moved more than a few mm in comparison to a standard GT.
The difference in those wheels is also width, one is a 7 et40, one is an 8 et20.
I dont think you can say et20 is on the limit of srubbing. a 10 inch wheel et30 isnt going to fit, but thats within the et20 limit!
Its all relative to do with the width and offset, as that is exactly what offset is. The relative distance between the centre of the wheel, and the mounting face on the hub.
Aaaaaaaaand breath.