In what way would exposing the true microcoded out-of-order etc nature of the beast benefit certain tasks?

Better control over the async nature of the hardware is part of what makes GPU kernels efficient, but I'm not terribly sure the same thing would be the case on the other side of the PCIe bus.

But even before you get to out-of-order/speculative execution, I think most languages lack good (i.e. non-intrinsic-based) support for wide registers or anything SIMD related. I know C++ and Rust are both working on this

It's a good and interesting question, why is it important whether or not it will "benefit certain tasks"? And how would we even know if we haven't tried it?