To start, the difference between the blanking plates with holes and without holes, is to allow some air flow, so an error code is not detected. I believe the EGR valve does not have any sensors to detect air flow, it just opens or closes or gets stuck!
True - there's no feedback to the ECU about flow from the EGR valve itself.
So, the error codes are detected by the map sensor?? am I right? the map sensor is a dual sensor which reads temperature via the bulb bit in the cage and pressure via a little hole just below the cage. So if there is no airflow into the manifold, the map can sense this, either via a change in temp or pressure or possibly both. The map expects to see some airflow at low revs and not at high revs ...is my understanding of how the egr works.
MAF sensor I believe, not the MAP.
ECU expects to see a drop in clean airflow when the EGR valve is open, hence looks for a step change in the airflow being reported by the MAF sensor.
Manifold pressure should remain relatively stable, as the VNT system on the turbo should adjust to maintain the same manifold pressure independent of EGR operation (it's a separate control loop I believe).. Since the exhaust pressure in the manifold is nice and high, the VNT system lowers the turbo output to maintain the same manifold pressure, hence lower clean airflow..
Fitting restriction plates works because it puts a restriction/pressure-drop in the EGR flow, so you get less diesel fart in the intake, hence more lovely clean air to maintain the same manifold pressure... But that hole is enough that the ECU recognizes a step-change in the MAF reading between open/closed..
Your situation (no EGR error, but down on power) would indicate MAF problems rather than MAP... It's worth popping the MAP off and giving it a clean up just because it's so easy to get to, but I'd be looking at the MAF straight after it that doesn't fix it.. In fact just try disconnecting the MAF sensor and going for a drive... It'll idle rough, but if you'v got the mid/top end power back then that's your problem.