CAN physical layer fault conditions Short circuit to battery on CANH and CANL lines analysis
Usually the midpoint bias and the internal transceivers are protected against hard faults such as these. Many also survive without issues to 60V levels or more.
You need to overrate the terminators only if you need to protect against one short to battery and the other to ground. The 120 ohm termination (or split termination in your case) only need to handle the full 5V dominant signal (it's quite a big resistor anyway, do the math). Unless you are using very old transceivers is difficult to estimate the ground impedance of the CAN transceiver since the circuitry is somewhat magic (i.e. self limiting)