Are we talking apples to apples here? No matter what changes you make to the ring and pinion in the differential housing, the change will be a global change that affects all the drive ratios equally. 1st gear will be affected as much as 5th gear.
The only way to accomplish the change you describe is to swap out the 5th gear assembly inside the transmission. Factory stock is 0.62:1 if I remember, and you would probably rather see 0.50:1 ala the F-body spec T-56 transmission. Such a swap can be considered, but the labor is much more involved than a ring and pinion swap.
The factory has researched gear ratios to death and they are designed to appeal to the average buyer and meet federal fuel mileage requirements. The idea that they would custom tailor a gearset that would only appeal to a tiny fraction of customers while requiring separate fuel mileage testing is pretty naive and short-sighted in my opinion.