I lean towards a separate review with some reviewers who work on other architectures, because I don't have access to most of the tier 2+ architectures for testing. Or, are you confident that it's no longer necessary?
Separate review is fine and probably necessary. It's not the case that all Tier 2 arches use clang because sparc64 exists (I keep forgetting it's still around in stable). This should be changed so all supported arches in 13 default to clang, but that will probably require testing.