If the rolling radius is the same then no difference, you need to check the wheel and tyre dimensions against an online checker, if the 15’s are low profile and 13s balloon tyres then there will be very little gearing difference. If the 15’s have a larger rolling radius it will give slower acceleration and I doubt it will have the power to pull a higher top speed.
if the tyres are wider then you also have a higher rolling resistance so the car would be slower and use more petrol.
Best off fitting an ST badge and powered by fairy dust sticker to add more torque to overcome it all (and drop the back seats)