It's fairly easy to make them non-brittle, because it's very easy to rig a testing framework to run both the hand-optimized "jet" and the pure code, and compare them.

Standardizing performance is a subtler and more interesting point. It's a fairly safe bet that anything in the kernel that needs to be is jet-propelled. Above that layer, who knows? Good old normative text may handle it.

But in general, the feeling should be like the difference between hardware GL and software GL. The developer doesn't see it and the user sees it only as fast versus slow.