You can definitely go bigger than 275 on a 18x9.5" wheel. Stock class autocrossers with RX8s and Solstices with 18x8" wheels run 285/30 all the time. Most 285/30s are rated for use on a 9.5" wheel. It might not have the best contact patch on a wheel that's on the narrow side of its rated range, but it will fit on there and be safe.
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