I've done what I can on my own to figure out what to use with Xcode/clang on the iMac Pro for hardware-specific optimization, but there isn't much information.
From sysctl(8) I know it's Family 0x06, Model 0x55, Stepping 0x04 (the 18 core 2.3 GHz aka "W-2191 B"), which is apparently "-march=skylake-avx512", but there aren't any built-in options to set that or something similar directly in Xcode.
Is there more to tune?
I'm wondering about this too. There doesn't seem to be any support for skylake-avx512 in Xcode or clang, and I've also seen one report that claims that the AVX512 instructions are disabled in current versions of macOS ?
They aren't disabled, and checking with sysctl(8) does reveal them. You can also test for the individual AVX512 features with macros when you turn it on (like -march=skylake-avx512):
#define __AVX2__ 1
#define __AVX512BW__ 1
#define __AVX512CD__ 1
#define __AVX512DQ__ 1
#define __AVX512F__ 1
#define __AVX512VL__ 1
#define __AVX__ 1