fairly common issue on all modern diesels this, not just Alfa, that its coming in so late strongly implies sticky/stuck VNT or a split or tear in the little vac hose that actuates it.
Maf is a strong candidate also as is Map sensor..cleaning the MAF on these is a bit hit and miss sometimes works sometimes doesn't. if you do replace it must be a genuine bosch item or the car will reject it in short order.
ok now to the location,from under the car you should be able to see the the actuator, its a circular steel arrangement with the vac hose coming from out of the middle... reach around that and there is the actuator rod..you should be able to move it.
here is a youtube link of the turbo off the car, should help you to visualize what you need to do do and what it look like..
hope that helps
I refer you to my first reply where the second thing I said was ''split in the little vac hose''
I don't often (ever) quote myself, but I had this happen after some work at Autolusso about a year ago, the hose was old and brittle and the lads had disturbed it when replacing the clutch/flywheel ..hard to track down as it presents like Maf/Map EGR and often you get codes that are confusing as they will be based on the ecu seeing a discrepancy between expected/actual airflow, to further confuse the issue exactly the same symptoms appeared when my MAF died about 6 months ago...took Loz an hour to sherlock it through and he knows these cars inside and out.
I spoke to Jamie porter (Alfa workshop) about this a while ago and he said that Lambda failure can also present with very similar symptoms so diagnosis is in fact easy but frustrating as you have to be patient and go through all of the power train sensors and mechanical's methodically clearing codes as you go.
in order of most likely failure points
2) vac hose to vnt actuator
4) boost leak, lower hose most common as its getting road crap on it, but it can be the upper hose,intercooler itself or even very occasionally a cracked inlet manifold especially CF4 as they are plastic.
5) VNT solenoid switch
6) MAP sensor, not as common as the MAF but still needs a clean or replacement periodically.
7) stuck variable vanes on the turbo
8) lambda sensor
9) coked up or blocked inlet manifold, if its CF4 swirl flaps begin to come into play as well here..
This will fix it most of the time but very occasionally someone will go through all of this and find its terminal with the turbo or there is a problem with the fuel pressure,either the fuel pressure sensor or a problem with the high pressure pump, or the injectors...
anyway, glad you have found the cause
now you can get on with enjoying your GT