is there any smoke out the back when you accelerate from low down?
if so this would imply some boost leak somewhere , or VNT vac hose as said previously, just because it operates freely on switch on/off does not mean it will be operating under load, I would change the vac hose just to be safe.
then onto the MAF, was the replacement genuine Bosch? if not it could still be an issue as these cars do not like aftermarket mafs, next onto the MAP sensor , if cleaning has not solved it then a new one may be in order, Lambda sensors can also present exactly like maf failure on the JTD engine, good article on the alfa workshop website about this..I would look at the vnt actuator solenoid as well.
and finally the Garrett calibration parameters on these turbo's is actually quite wide as the flow bench software used to calibrate them gives either a green or red light, however ''in spec'' can be anywhere from 1500 rpm spool to over 2k, and its been said on many occasion that some JTD's ''just don't pull from down low'' meaning they are on the upper end of a ''pass''
if you rule out all of the usual suspects in the power train it will then come down to VNT calibration and you will have to decide whether to take the turbo off the car and have it looked at.