will Factory Cup wheels from a '93 RSA fit on a 88 Coupe? what modification would I have to do? How wide are the Cup wheels (aka C2 Turbo wheels)?

 

7JX17 front and 9JX17 rear.
Offset of the RSA is 52.3mm front and rear.
In the USA you will most likely find that the wheels have a 55mm offset here in Europe 52.3mm. I have also seen 55mm front and 52.3mm rear. My set of Turbo wheels are 52.3mm front and rear.
Will they fit your 1988 Carrera. Not without spacers they won´t and I suspect some rolling of fenders may also be required.
Ciao,
Adrian
911C4