Widest tire 19 CTS-V

If the wheel clears, the additional 10mm of tire shouldn't be a problem. The tire diameter will also increase (by 30% of 10mm), but that's just a few mm so it's not much of a concern. And the extra 10mm width is going to be split as 5mm on the outside and 5mm on the inside. If your wheels only have 5mm clearance to begin with, I'd argue they're too wide and the tire is the least of your concerns... If your wheels have more than 5mm clearance, the tires should be fine.