[aland]

Today I swapped the turbo with my spare one.
Although the actuator moved the VNT through its range, with the actuator removed, and both turbos sitting next to each other, there was a noticeable difference.
The one from the car felt gritty to move the VNT, and if you rocked the lever side to side it got stuck.
The spare one was smooth, and doesn't stick.
The circlip holding the actuator rod is very fiddly, I'd say impossible to remove and re-fit this on the car to test it.

I removed the bolts holding the exhaust chamber VNT together, but it didn't want to come apart, and I didn't want to risk damaging it.
With the replacement turbo, no more limp mode, and the VNT vacuum sits around 10 in/Hg as you hold your foot on the floor, when before it dropped to zero - confirming that the reduced vacuum IS reducing boost as requested now, when before it was over-boosting and the vacuum was being reduced to zero by the ECU as it desperately tried to reduce the boost.

The official procedure to remove the turbo involves removing the RHS driveshaft, but I found by removing the heat shield over the steering rack (gained a few mm) and a bolt from the rear engine mount, by then jacking the engine a little opens enough gap to get the turbo down and out.
A second person to help is really needed with this job, especially when trying to re-fit the turbo, against gravity.

Based on my findings, I'd suggest that the VNT mechanism test with a vacuum pump is only useful to prove the actuator works, but if the VNT is worn / sticking, it won't show it.