From the website "mellonhead" quoted, off the page linked to the installation of the boost controller that he posted, a bit about how the sequential system works: 'The only way for the ECU to enforce boost control for the whole system now is via the WGV of turbo#1.'
Basically the number 2 turbo wastegate isn't a true wastegate, it dumps excess pressure back into the system rather than venting it, the number 1 wastegate is the only method of boost control there is, hence needed a restrictor ring to build up backpressure to slow the turbos down for bpu, if you ran an unrestricted exhaust then you just blow the number 1 wastegate wide open and still overboost as its trying to vent pressure from two turbos when it was only designed to control 1..... at least this is my understanding of it, hopefully Ian C will come along and correct me if im wrong!