Wheel spacers: For looks, for handling or both?

Guard Dad

I'm seriously considering getting 15mm front, 20mm rear wheel spacers but I've got some questions. My Z is a daily-driver, it has the sport package and I will not be dropping the car or changing the wheels.

I know that in theory the wider track provided by the spacers should improve handling but was any actual improvement noted or is it strictly an appearance mod?

The 15/20 combo seems popular although some folks have gone beyond that to 20/20 or 20/25. I'm a more-is-better kind of guy but I don't want the wheels hanging out in the breeze plus I am considering 275/35 front tires and 305/30 rear tires when the time comes for new tires so I wouldn't want spacers that are so wide that the tires are hitting the fenders. Any thoughts?.