I'm doing the same thing with my widebody 3.2. No mine's not a turbo car but I'm doing it for the same reasons you are. 8&9x15 Fuchs with 205/55 (or 225/50) fronts and 225/50 or 245/50 rear tires.
Here's an example. It appears to be on 7&8x15 Fuchs with 225/50 and 245/50 tires:
IMO opinion anything smaller than 24" tall looks a bit funny...but...it makes a noticable difference in gearing. If you go with 205/50 front tires then consider 225/45 rear tires. These are ~23" tall. But even 24" tall tires make nice gearing changes vs. 25" or taller tires.
I'll probably keep a second set of wheels that are 17's so I can switch back and forth when desired.