Eric is correct, increasing the voltage of the transient clamp will significantly reduce the time for solenoid response/recovery. If u consider the use of solenoids in automotive fuel injection, the on/off cycle is critical and must be minimised otherwise larger injectors are called for. If the drivers of these solenoids can tolerate a 60V transient max. Then perhaps a 40V transient clamp will work well. Clamping down to 12VDC is unnecessary and can extend the 'freewheel' time by several milliseconds.