A two step is used for manual transmission vehicles, it allows you to set a launch RPM that is activated when the clutch is depressed so that you can open the throttle at the line and hold a specific RPM / boost level without having to pay attention to the tac or feather the throttle. I don't see how it would be beneficial on an auto trans unless you had a trans brake. I think as stated above, spooling the turbos on the foot brake would be far more beneficial and won't reek as much havoc on a stock transmission as trying to launch 5500 lbs off of a trans brake at full boost.
Here's my Shelby bouncing on the 2-step. It's set at a low RPM in this video so its relatively mild sounding. As you'll see by the big old flash coming out of the muffler, it's NOT GOOD at all for cats. It will also destroy some mufflers on the quick as well. I've seen pretty descent quality mufflers burst at the seams from two step launches.