Have you tried running with the egr valve disconnected electrically, or better still, blanked. The P0235 fault might mean that it's being held open all the time. With too much exhaust gas getting into the engine you will have poor combustion, smoke and power loss. Your VNT system, which seems to be working, won't have much effect at low revs because the engine won't be able to generate the gas flow to get the turbine spinning, regardless of the VNT position.
(Running with the egr disabled will generate a code but iirc you won't get limp mode.)
Edit: Beg pardon Kenclarke, I misread P0235 as egr circuit failure, instead of boost sensor circuit failure. Forget the above. Have you checked the inter-cooler for leaks?
Last edited by R.T.; 23-05-16 at 20:01.