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What's the downside of installing the 14A clamp for the FET driver circuit?

No downside that I know to be real. I have been running Low imp on four cylinders and have not seen any current draw even close to worry about. And with three per channel there is still plenty of room even with low imp injectors.
But the board is designed by an 8 cylinder guy an he needs to provide protection at that level. Then the clamps may come to value.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.