Currently nothing in MSVC STL calls the allocate_at_least member function of an allocator except the mandatory call in allocator_traits, which means that even if one adds allocate_at_least with the expected optimization to a provided allocator type, the behavior of STL doesn't change at all.

libc++ has made use of allocate_at_least via LLVM-D122877. I think MSVC STL should also do so.