There are just more performance tires in 15s than 16s, plus overall 16s weigh more than 15s - so you get rid of rotational mass, which helps accelerate the car quicker, and help he suspension and brakes work better.

For every 1lb wheel weight you drop it works out to about 9lbs static body weight. So 15x7" Kosei TS which weigh 10.5lbs compared to 17x7" 18lb wheels equal difference of 7.5lbs*9=67.5lbs you just got rid of off your car (static body weight).