The swingweight about the wrist pivot will be different depending on where the player grips the handle. If one chokes down to the bottom of the grip, the swingweight and MgR/I will be higher; if one chokes up, these values will be lower.
Federer and Nadal, for example, hold their racquets at the bottom of the handle (little finger just barely on the grip) on groundstrokes, making their MgR/I numbers lower than they would appear.
I think it would be helpful to establish a reference handle position for comparing MgR/I numbers, i.e. little finger just above the taper of the buttcap.
It would also be helpful to have a conversion for other grip positions - up or down the handle.