FWIW, I just did a quick test using the leftover piece of fork steerer I have and a spare stem. Prior to clamping, the ID of the tube measured 23.65mm perpendicular to the stem slot and 23.8mm parallel to the slot. After clamping with a reasonable torque, 23.65 became 23.7 and 23.8 became 23.6mm. So the steerer definitely ovalizes as it is clamped justifying the effort in my mind. My insert piece goes from being a slip fit to not fitting at all, prior to and after clamping respectively. Bonded in place, it should be offering just the support I expected it to.