Meh, I wouldn't worry about wiring a radio directly to a starting battery. It's only /potentially/ a problem if you have the radio turned on during cranking, which I do all the time anyway... The power in my FT-8800 has a TVS across the 13.8V that clamps spikes to 18V (Yaesu uses a P6KA18, good to 600W and 1ps response). The constant 13.8V is fed to a 5V linear regulator that is fairly heavily filtered for the microprocessor, which is always powered so that pushing and holding the power button turns on the switched 13.8V for the rest of the circuits.
The output FETs are RD70HVF1, good to 20V gate voltage, and the driving FETs are 2SK2975 (Yeasu uses these all over, the finals in the FT817ND are the 2SK2975 as well, which are driven at 5W over 160m up to 70cm up to 16V), also safe to 20V Vgs.
Honestly, these radios are designed to survive this sort of thing, I'm not too concerned. You know the heat/cool cycles and the dust/vibe/shock we hit these radios is just as bad for them as the power noise. Though you ought to wire them to the battery and not a secondary tap. The battery is a VERY good filter and the power is actually very clean tapped directly on it.
"Silent enim leges inter arma." -- Cicero