Can statistical models be used for multicore WCET analysis? Is it sufficient to be x standard deviations away from the mean?

There are some promising paths for doing WCET analysis for multicore systems that utilize statistical approaches. No statistical methodologies have been well established in terms of certification, so your chosen approach should be agreed upon with your certification authority.

Execution time in a multicore system does not follow a normal distribution, so the standard deviation is not meaningful as a statistical measure. This is due to consecutive executions of software not being independent from each other. One cause of this is it not being possible to know or control the initial state of deeply embedded hardware elements, which may have been left in any number of states by previous executions.  

The use of statistical models for WCET analysis is discussed in detail in the white paper WCET Considerations in MACH¹⁷⁸ Foundations.