If its the ECU causing the boost to limit YOU WILL KNOW ABOUT IT in the form of very loud backfires!

If you have the bleed valve fully un-screwed (you brave man ) and thats all its reaching, you either have a weak actuator and/or damaged wastegate.

My turbo had a cracked wastegate and i actually had the screw from the bleed valve in the car with me, it would peak at 10 and tail off and hold 6