exposed ABI

ABI is not something that gets "exposed" or not.

it will always be ambiguous as implementations are hard to compare across languages

Correct.

in the transition for x86_64 they were seeing up to 70% increase in compile speeds

And that was a part of what I was hinting at, because you get >>70% speed-up with Cranelift in most Rust projects. But in either case, faster code generation is not free lunch, hence the mention of comparable runtime performance of generated binaries.