For a stock car with a new-ish wiring system, there will be very little effect... if any.

For a modified car with a powerful sound system, having the extra grounding can help prevent the lights from dimming too much with every "whoomp" of the subwoofer...

In terms of power... it's hit-or-miss. To make any appreciable power on a stock car, a grounding kit has to be specifically tested and installed on that vehicle... preferably on the dyno. It's hit or miss, really, and the extra 2-3 hp is not worth all the hassle of all that set-up time.