I am a bit of a noob with wheel specs, but this is my experience so far...
I was running 18'x7.5 on my car with a ET42 (not lowered). I now have 15x7 with ET29 (lowered on Eibach pro kit - 25-30mm drop). With the 18s running that offset there was some scrubbing on the shocks due to the width. It was scrubbing on the arches on heavy bumps too. The lips need to be rolled. But now I have enough space in the back and little poke on the front wheels.
The rear arches are quite flared on the Ibiza, so you can go pretty wide. I am looking to put some spacers in the back to even out the look compared to the front. With ET29 on the back I still have about 10-15mm to go until the rims are flush with the arch. So I might consider a 15-20mm spacer and roll the lip on the arches.
Nice choice of wheel, I had these on my Polo.
MK6 Golf GTI ///Adidas | 9N Polo GTI | 96 Seat Ibiza 8v GTI
Quote of the day, from my wife.... "Stop washing your car... it's going to rain anyway"