Firstly, what N75 valve are we talking?
Normally the N75 will give you a good spike, and then fade away quite quickly. The reason being that the map that's on the car controls it. You can see what's going on by looking at the N75 duty cycle on a log.
You need to understand how the N75 actually works for openers.
Boost controller - Wikipedia, the free encyclopedia &
What the N75 valve does... - SEAT Cupra.net - SEAT Forum.
When trying achieve max boost the duty cycle will be at 95% - this means the N75 is effectively closed. As the boost gets close to requested, this will drop down to the 70s and then down further. This means that the valve is opening and is allowing the boost to fade away as it opens the wastegate on the turbo via the actuator.
An MBC doesn't suffer from this. The ball and spring mechanism on the MBC will not open untill the pre set boost level is reached, and will keep this boost level for as long as the turbo can produce it. As the pressure drops because the MBC is open, the ball & spring will close the valve again, allowing boost to come back to the preset level. Good info here:
Intro to Boost Control
Limp mode, as you know is a self protective safe mode effectively. Brought on by many things, boost being one of them. The engines have a MAP (Manifold Absolute Pressure) sensor - so the ECU knows what the boost pressure in the charge system is. The ECU therefore can compare what boost it asked for, and what boost it has. If there is a difference between the two of more than 250 millibars, it will assume there is a problem and hit limp mode.
Also - if fitting a manual boost controller, it is usually worth doing the N249 bypass. This is another way that the ECU is able to vent boost from the charge system - it opens the DV at the ECU's request.
Even with the N249 disconnected, the ECU can still have you though. I have no N75 and no N249. This means that boost is completely beyond the ECU's control. However, being the clever little sod it is, when (and I do occasionally on the MBC) hit limp mode, it cuts the spark to the coil on cylinder 1 to stop it being driven...
Little git.