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It likely is a result of higher torque and rotational speed with the GT500. The current and last GT500's are supercharged and make a lot more torque than the GT350. They are also more powerful and capable of higher top speeds. The driveshaft is after the transmission so its rpms are tied to the tires (with a speed multiplier equal to the rear gears/axle ratio). The GT350 may rev a lot higher, but the GT500 driveshaft should see higher rpms as a result of more tire rpms because of the higher top speed.
True, although ultimately the speed of the driveshaft will be limited by how fast the car can go and the wheels can turn. I think I read that the GT350 tops out under 180 mph whereas the last GT500 topped out over 200 mph and I'm sure this one will as well so higher rpms for the driveshaft (a bit offset by the taller wheel and tire like you said).