Before my 17" Turbo's,I had some 964 C2 wheels fitted,and those where 8x16 et52 with a 25mm spacer.
They as well fitted without any problems or tyre rubbing in the rear wheel arces.
Tyres where 215/40x16 .... what kind of size are you putting on you're 17"'s ??

I also had some 8x17 et70 ( ) at one moment in time,and those tyres were 225/45x17