Rust already uses MIR-based libraries for LTO, and uses thinLTO by default.

ABIs won't help automatically. The overhead comes mainly from generics and macros. Cross-crate monomorphic generics and AST macros are impossible to express in an ABI. Turning them into polymorphic usually wouldn't remove the many abstraction layers that need to be peeled off, and only make them cost at runtime too.

So far MIR optimizations were helpful. Rust can simplify high-level MIR quicker than LLVM can simplify the same code after it's been lowered to lots of low-level instructions.