If you put a fast recovery diode eg BYW29 or BYV229 across the output of the BTS-441 which is driving the ignition-coil you'd get a better control of the coil. Put the cathode to the plus side and the anode to the ground, but I guess you already knew that.
I know that the BTS-441 has a diode built-in but I doubt it's fast enough to kill the EMK of the coil. When driving coils with FET's, especially at high frequency I allways put such a diode in.

maybe this helps.