If you want the car just for the looks.........it's OK.
I use 8,5 x 18 ET 46 wheels with 225/40/18.
I'm 15mm front and 10mm rear lower than standard....and I have some small issues when fast cornering.
I think 30mm is too low...
Remember that the GTI is already 10 to 15mm lower than the other Golfs (R32 not included) and that the OEM wheels are 7,5 x 18 ET 51