For the cost of machining, new valve seats and new valves, run an extra .5psi of boost if you must. Going 1mm larger valves on a multi valve 2.5L 6-clyinder engine is not really worth the effort in my opinion, especially for turbo applications.
264 cams are very mild, a step-up to 272 will cost no extra money and with modern ecu's you wont suffer any drivability issues and it will open up the top end.
JA
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