The 5.5 thing you keep hearing about is what happens when the front wheels are not moving. If you just go out and start your car, leave it in park or neutral, and floor the gas pedal without the car moving, the car will not reach redline.

This is not really a big deal to most people, but if you are trying to do a dyno run on your car the front wheels are typically stationary, causing the PCM to not allow you to reach true redline (6000RPM or higher).

If you're on a 4 wheel chassis dyno this is not an issue.