Well, Big Girl still has a boost problem.

She has the new MAF sensor (plus new air-filter and thorough clean out before it went in)
New air-pressure hoses (the lot)
A new Boost solenoid (apparently the new version is an upgrade from Nissan) - as recommended by the mechanic who checked for codes
I removed the shim (trying anything now!)
Installed a new fuel filter


The mechanics recommendation: no idea - take it to Nissan...

and now I'm ready to take her to a 'professional' dyno mechanic.
I asked around, the word is: DON'T take it to the local Nissan workshop.