In numerous discussions with the FGT engineers while at the Detroit rally and Ford Centenial I have come to the conclusion the team reviewed all aspects of our car components critically. The brake designers fought for every milimeter of rotor diameter for swept area which was traded against weight. The team was VERY cognizant of vehicle weight. To have such a large weight on our transaxle shift lever, it must have a very specific (and necessary) purpose. To think this lever arm weight slipped by under the scrutiny radar is unrealistic.
I believe as has already been posted the weight is most probably for shifter feel, however there could be another reason. The arm may have a dynamic excitation somewhere in the operating range that the team thought it best to damp. Thus if you remove the weight the arm may vibrate (or be excited to vibrate) with undesireable consequences. Just a thought. I have seen similar weights added to other automotive parts for just this purpose.
I know we are all "tinkers" and think we can always optimize our GT components, but give Fred's engineering team a little credit. They DID their homework on this car!