> Major compilers (LLVM, GCC, Intel) are able to vectorize real world code to a useful extent.

Try compiling this with recent gcc:

    uint8_t dumb_checksum(uint8_t *ar) {
      uint8_t ans = 0;
      for (int i = 0; i < 320; i++) ans += ar[i];
      return ans;
    }

You don't get satisfactory results. I, at least, don't have a clear mental model of when gcc will do something reasonable and when it will generate a large amount of bad code.