This is pretty good. Since the voltage depends only on the inductance of the coil and the instantaneous rate of change in current, which is due to the speed of the distributor rotor and physical characteristics of the coil, so voltage doesn't seem to play a part provided there's enough to charge the coil. When
charging, the energy stored depends only in the current and inductance, so the only problem would be if there wasn't enough time to charge the coil or voltage was so low (less than 9V according to the first link) that it wouldn't charge at all.
Since engines are designed to operate to two or three times the speed we usually drive at, we should see missing at high rpms is there is a problem with enough energy getting to the coil, but given the much longer charging times if your Previa still runs on 12V I think it's safe to say that there's enough voltage at 12V to charge the coil.
Hotter and cooler spark plugs are something that aren't needed unless we're talking about an engine operating outside it's designed range. For instance, changing the ignition coil can lead to higher voltages and a stronger spark so to speak if colder plugs are run in order to minimize issues w/ detonation at higher power outputs w/o having to deal w/ poor running from fouled tips at idle.
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Originally Posted by Wikipedia
For normal use, the selection of a spark plug heat range is a balance between keeping the tip hot enough at idle to prevent fouling and cold enough at maximum power to prevent pre-ignition leading to engine knocking.
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