The point of interfaces is to decouple caller implementation details from callee implementation details, which almost by definition prevents optimization opportunities that rely on the respective details. There is no free lunch, so to speak. Whole-program optimization affords more optimizations, but also reduces tractability of the generated code and its relation to the source code, including the modularization present in the source code.

In the current software landscape, I don’t see these additional optimizations as a priority.