US spec cars are slower, 180HP on the crank on GTS cars, the tranny saps the power away, only about 165-170 for the manuals and almost 160 for the autos. Manual shifts help keep times low, out the two extra gears in the manual can make a big difference depending on the driver's skill.
While a shift kit would help, with a sportshift, it's not that big an issue, since the driver manually determines the shift points. However, the shift kit would also add quicker valve body transitions, so it shortens the shift timing, makes them harder, sort of like a short shfter on a manual car.
Torque convertor to raise the stall speed would be cool, think of the convertor as the clutch pedal in a manual car, the higher you stall before dropping the clutch, the more momentum will be built, and the more power you get, overdo it and you can stall the motor or worse, wreck the gearbox. the GTS auto's have a stall of about 2000 RPM, bump that up to a 2500 stall and you will notice a difference, I don't reccommend anything higher than a 3000 stall, new axles would help, and a shift kit would be a must.
Heard a rumour of a LSD kit from phantom grip for the auto, however, I shudder to think of the difficulty in installing it. If you can get one, I suggest it for the most hardcore, serious racer. probably isn't cheap either.