I don't think there's going to be any performance difference at all compared to the basic case, which actually is another reason why this is so good. Ultimately, all these struct just hold a double, which means that in memory they're exactly a double and nothing more, there's no type tag or anything like that. And all the code is static so there is no vtable, just the double value. The compiler does the rest, not the runtime.

(Still, I'd love to see newtypes in C++)