Anyway, business logic is a perfect example of stuff that should go in Policy: functional, side-effect-free code (but not necessarily tidy or easy to read) which computes a value from a set of inputs. Like the parent says, effort spent optimising here may end up being a poor investment if the requirements change every year.
Mechanism code, OTOH, generally ends up containing all the side effects. It's harder to test so it tends to be dumb and, therefore, less susceptible to changing requirements. Optimisations here can pay off because the code lives longer.
Really though it all comes down to tests, tests and more tests. Automation is an asset and test cases outlive everything so make sure they work against the interface, not the implementation.