I think this is the overhead for mitigating by adding additional code (a branch on overflow that can be predicted as not taken), rather than the cost of lost optimizations that could have been used by assuming that UB does not occur. So a useful number, but measuring something else.

Although perhaps it can be used an an upper bound on the boost provided UB-free optimizations? That is, the overflow checking has both a direct cost, and prevents certain beneficial optimizations. Thus we can assume that the optimization benefit (on SPEC CPU 2006) is something less than 30%. That is to say, real, but not enormous compared to an algorithmic difference.