It's the power company's job to provide you with good quality power that meets the requirements of your appliances. If they're not doing an adequate job, get on their case!
However, if your heat pump says it can take 258VAC, then it can take 258V without any issues. It can probably take 270V or more, but that's akin to overclocking.
Higher voltage means higher compressor RPM and higher instantaneous power draw, but since the compressor provides cooling commensurately faster, I really don't see how it could have any impact on your average power draw.
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