A snubber diode is the wrong approach here as its purpose is to dissapate the energy from the coil to protect the transistor. In a transistor ignition system you want to transfer as much of the the energy as possible to the spark plug then turn the transistor on as the plug quenches to re-saturate the coil.
Capacitive discharge ignitions popular in the early 70's used a different approach . They use a power inverter to charge a capacitor up to about 400 volts then triggered an scr to fire the spark plug. When the scr commutated the remaining energy in the coil was returned to the capacitor via a snubbing diode reducing the load on the inverter.
The original Kettering Ignition using points has a capacitor across the coil to reduce the arcing at point opening. This cap also extends the operating range of the ignition systm by tuning with the coils inductance at a fairly low Q thus extending the top end of the frequency range.
I have not studied current ignition systems but the magnetic design of the coil itself woud be a significvant factor in the total system design. Terry