Good question. That's the point of this thread. My other car ran 21 psi with no EBC so either there was another way of controlling the boost level, or the actuators were set at that level when my tuner installed them. Either way, the boost would trail off in the last couple of thousand rpm. So I had the EBC fitted and not only was the peak boost maintained all the way to the redline, but it made boost noticeably earlier at all points in the rev range and was MUCH nicer to drive. My conclusion therefore is that an EBC is very worthwhile and that (probably / maybe) even though the actuators had been modified they weren't doing the job properly. I'd guess that this is because the actuators are springs and that springs do not open and shut in a switch like fashion, but begin to open partially before peak boost is arrived at and then leak from there on.
I'd welcome comments to clarrify this.