Unless I'm reading this wrong, I think the whole "square/circle" argument comes down to whether or not a mouse player would be able to push both the X and Y axes to their limit at the same time, which any joystick user can obviously do. In other words, your standard joystick, being a "square" example, has four "corners" for limits; you can see these in the standard Windows game controller calibration screen. In each of those corners, both the X and Y axes are at 100% motion; translated to in-game, this would allow your fighter/turret/whatever to move most effectively in those directions. With a "circle" model of motion, these "corners" do not exist; when attempting to move equally in the X and Y directions, instead of getting the 100% generated by a joystick, you'd be getting (100/squareroot(2))%, via simple geometry. This produces a limit on how fast you can actually move the reticle and could put the mouser at a disadvantage.