Well... I'm basically not even using any delta time nor anything, capped FPS with VSync and left it like that.
Then you're doing something quite wrong in your calculations...
Also keep in mind that this can be quite a bit 'dangerous', because SFML can't guarantee that VSync gets activated, because one usually can force VSync on/off with the driver.
Should fixed timestep actually fix it?
Probably not, since it's a problem in your math (I guess), but it's better in any case.