You want more side wall when you drag race...
Each car is different on how much or little tire/rim you need to drag race.

As an example...... a 16x8 rim with 245-50-16's with hook better than say a 275-40-17 on a vehicle with 300+ foot pounds of torque.
A viper runs X/50/x on the back and X/40/x on the front because it needs the extra sidewall to help it hook better.

Chances are with your BMW you will run better with your 15 inch tires...