question on evo boost control

I had one guy tell me that the boost is completely controlled by the ECU (activating the wastegate) to keep it where the ECU thinks it should be.

Another guy told me that I can just add a manual boost controller and I can set the boost exactly where I want it.

Who is right? Does the ECU control the wastegate and under what parameters?

The reason I ask is that I hit max boost (21psi) at about 2900 RPM, then it pulls back to about 16-17 psi for awhile, then it slowly creeps back up over 20. Even if I am cruizing at 4000 RPMs and hit the gas, it still peaks at 21, backs off to 17, then creeps back to 21. I want to know how to keep it solid at 20-21 at all RPMS.

Thanks!!
EVOlutionary

I have heard that you can control it with a manual boost controler. If you need any parts, you can get really good ones from RPMmotoring.com (they arnt active right now, but they should be soon).