Regardless, apart from pathological integer math cases, these things have little to do with the cost of overflow checks. Rather, it's about the followup optimizations the compiler can do after the UB frees it from a restrictive assumption. The trivial common examples (optimizing x*2/2 to x for integers, etc.) don't really capture the impact of these things very well, because you can't demonstrate it in a oneliner, but you can derive representative examples pretty easily if you take tight-ish loops (>16 independent loads, some SIMD-able compute, a few dependent stores at the end) and switch the index types. The vectorizer is often easy to defeat if it can't assume no overflows, the cost of the check itself is irrelevant.
On your last point, I don't think compute intensive kernels are "self-evidently more important" than anything else. I think there are usecases - like scientific computing, device drivers and embedded software, to name a few - that often benefit from prioritizing programmer control over any safety net, and FORTRAN doesn't cut it for many of those uses. There are plenty of languages that give you all the safety you could possibly want. C/C++ have never been about that and I don't see why that should change. Use the right tool for the job, by all means advocate for people who don't need this level of control to use something "safer", but the notion that every tool should be blunted to some common denominator just makes no sense to me.