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Stock is around 11psi, 15% is around 14psi. I've seen car on stock internals into the 25-30 psi range running turbos. 8.5:1 compression isn't high at all and the motor is built like a tank.
That's the Cooper. It doesn't have any boost and the engine and transmission won't take it. The Cooper S is 8.5:1... I don't know about the R56 though which is probably what you're looking at.