The mitsubishis use 6V glowplugs with a big resistor to take up half of that voltage. The reason is that they can put a full 12v over the glowplugs for a couple seconds to heat them up really fast, and then the solenoid switches over to put the resistor into the circuit, so that the glow plugs don't burn out.

I would think that your 12V plugs should be fine, provided you aren't putting that resistor into the circuit... They'll just take a few more seconds to heat up than the original mitsubishi design.

So the other question is: how vigorously does the starter turn the engine? Maybe a more burly starter and or battery can help? Maybe a lighter weight oil?