Smaller tires and acceleration

Any idea of how much time I can gain by changing my 17" tires to 15". The car has 200 HP and curb weight of 3400, Speed limiter at 180 km/h so I don't care about top-speed.

The main advantage of going with a smaller tire/wheel is a reduction in unsprung weight (depending on the wheel/tire). If you can reduce the total package weight of the tire/wheel on each corner you can expect a little improvement in speed off the line. If you can reduce your tire O.D. and maintain the load rating it will give you a slight advantage by changing your gear ratio on the back end of everything.