if you want to go fast in a straight line, DRAG race, first off keep it on the track, and on the track you wont need to downshift, downshifting is ok, as long as you dont go too low a gear, where the RPMs will exceed the redline for your car (about 7k for an LS, and 8k for a GSR)
for DRAG techniques, since we have front wheel drive cars, I would not recommend going over 3k RPM during a launch, with slight feathering of the clutch until you have traction. If you drop the clutch entirely you will lose traction instantly.
This is assuming you have a stock or mild setup.
without any electronic traction control on street legal tires.