i think if you're trying to go fast in a gts auto, flooring it works better than sportshift mode.
in sportshift, even tho you're telling the car when to change gears, it takes about a second from when you push the "up" button until it actually CHANGES gears. if you're trying to get the car to shift at redline, you're gonna have to push the "up" button earlier (like around 6500 rpms).
if you're in full auto mode, and you're flooring it (assuming your engine is warm enough) the car will shift right at redline and you don't have to worry about the time delay between pressing the "up" button and the actual shift.
btw i have a gts auto and i've learned to love it
but i still wish i had a six-speed
my last car was an e30 m3 ... i miss shifting ...