You realize that your code generates the circle in a single call, while SFML has to provide each point separately, right? Although this doesn't mean that it is impossible to precompute stuff like your code does, it makes it more "ugly" by adding members to the class and managing states that persist across the calls to getPoint.
So, please start by the beginning: do you have a use case where optimizing this specific function show significant benefit? Or are you just sharing random thoughts about SFML sources?