I think some people are unclear as to what a "sigma" trip is.
Plateau sigma is not reached by doing inhuman doses of DXM. It's a state reached by prolonging the experience--i.e., you take a low second plateau dose, then a few hours later another low second plateau dose, and then when you're starting to come down, you take a third plateau dose. Allegedly, when you begin to come down from the last dose, you will be left in the "sigma" state. It's called Plateau Sigma because of the high potentiation of the sigma receptors and the fact that sigma is the greek letter denoting "summation" in mathematics--as in, you take a "summation" of doses to reach the state instead of one massive dose.
FYI: Plateau sigma is highly dangerous, and is pushing the limits of safety.
The DXM FAQ on Erowid.com is really fascinating and describes all of this in great detail. It's cheery rainy day reading.