Damage to the powerplant can cause drop in power or malfunction. Having a bit to spare doesn't hurt but if you can optimise it to 99% then its usually worth it provided you are running shields to prevent said powerplant damage.
If you are running a silent running combat I'd highly advise some spare power although without a shield that setup should be easy...
If damage is a concern, can also set some or all weapons or utilities to priority 2 and FSD to priority 3. Set it up so that in the event of damage, shields and engines then get charged over being able to shoot back whilst you're most likely wanting to run away.
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