The first thing that jumped to mind was the MAF (Mass Air Flow Sensor)
but having done what you have and the test that Russ has done on T4
this should have shown itself up.
You could try just up[lugging the MAF
and she if she improves.
If the Turbo is coming in OK and is working fine we are only left with a Faulty Injector or two but the balance of them should have shown up on T4
and if faulty or out Russ would have said so.
A leak-back test is still a good option if everything else has failed.