Good point. I seem to remember reading that on the 35d they are actually using glow plugs nominally rated at 6 volts to speed-up the preheat phase:

"Another feature of the reworked engines is the spontaneous preglow system. The
new glow plugs are designed for 6 Volt, but for an extremely short preglow time, it is
possible to overload them for a limited period, also the mass to be heated has been
dramatically reduced.
This new preglow system delivers approximately 150 degrees higher temperature at
the glowplug tip and occurs in less than 3 seconds.
The effect for the customer is that in the new engine there is no preglowing
necessary at temperatures above minus 5º Celsius (23º Fahrenheit). The maximum
preglow time at minus 25º Celsius (-13º Fahrenheit) is reduced from 9 to 2.5 seconds
compared with the predecessor."

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