You're under the misconception that running a mouse through an adapter does more than emulate the gamepad. It doesn't. The speed of aiming is still limited by the same constraints that the game uses for the gamepad because those are the only available parameters. There is no native mouselook for the mouse to unlock the speed and pixel-to-pixel precision it would have on a PC. This is one of my gripes about XIM3 marketing. One-to-one or linear control is true, up to the maximum turn speed allowed by the game, but pixel precision is still a matter of starting and stopping movement of the mouse in the same way one would use the joystick.
While a mouse can prove to be less temperamental than a miniature joystick, on a console this is entirely a matter of ergonomics and physical control. Higher speeds come from the player being able to handle maximum in-game sensitivity more easily than with the unweildy thumbstick, and precision from the fact that the mouse is more straightforward in handling where starting, stopping and changing direction of the hand are more efficient tasks than centering and then tilting the thumbstick to the proper angle, for the proper amount of time, until the turn is complete and the stick must once again be centered, all with only a single thumb. Any advantage offered is, to a greater degree, the same a player might get from a larger joystick.