Indeed. No UB is something no mainstream C/C++ compiler does (and maybe ever did). Benchmarking the performance impact will require basically rewriting GCC or LLVM, an insurmountable task, because the assumptions are so ingrained.

Though it might be possible to have more "sensible" UB. For example, assuming daemonic non-determinism for allocation address it is possible to turn stack storage into registers, without the need for an aliasing model. Benchmarking it has the same difficulty.