If your going with 255's you need the following.. And anything else you will seriously struggle to make them fit.. (bare in mind this is all on a Golf too) I dont know if there is much difference in arch clearances on the Cupra, I know there was with the S3..
* Heavily modified arches all round, cut away bumper support brackets, rolled lips, removed arch liners, pulled LOTS
* Wheels specs need to be 18 x 8.5 / ET50 (48 at a minimum)
* 1.5 degrees front camber or more, 2.0 degrees rear or more (bare in mind you cant get away with as much with a 255
* Also, im not sure on strut diameters of the different coilovers, mine are H&R's and Im running 1mm clearance on the front, 2mm rear..
If you do all of that then you will be able to get a 255 on..
Good luck