According to tire rack the smallest I was able to go was 16 inch for my 2014 S GT. Since the '14 and '15 models are essentially the same I would bet 16's are the smallest you can go. When I put my snow tires on it didn't seem like there was a lot of extra space with 16's so I don't think 15's will work. If you want I can try to a picture of the clearance between the brakes and the inside of the wheel with my snow tires on.
I have an iSport, which has the 16" steel wheels. I took a couple pics to (hopefully) demonstrate the clearance between the brake caliper and the inside of the wheel:
The wheels in question:
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Close-ups:
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I honestly wouldn't recommend going any smaller than 16". I don't think 15" wheels would fit over the brake calipers.
According to TireRack, they have no tires at all in 195/65R16, so I assume it was the available 195/60R16 (which is current $5 more than the 205/60R16), but that would be a little short in overall diameter.I went with 205/60R16's for my car. The selection was the deal breaker for the 195's but the price wasn't too different. I think the X-Ice Xi3's were $5-10 more for the 195's on tire rack.