I have recently stumbled upon Jedi's Trick and it looks promising for converting sensitivities between 3d and 2d games via minimising error over all monitor matching distances.
I'm not sure if this is a bug or intentional, however, whenever I change the raw input setting for converting from a 3d game to "osu!" whilst using Jedi's Trick, the mousespeed/sensitivity changes despite the value not changing for any other conversion methods.