Been having a problem for a few weeks now especially when cold.
The car will accelerate very sluggishly when floored from low revs (sub 1500rpm) and produces clouds of black/dark gray smoke.
It eventually picks up about 2500rpm and the smoke clears.
Initially I was thinking stuck vanes on the VNT, but decided to do a bit of investigating before condemming it.
First was a check for boost leaks & that all vacuum & boost pressure hoses were on and had no holes. All was in order there.
Next was a check of the VNT actuator by sucking on the vacuum actuator pipe where it goes to the VSV (control valve). This seemed to be moving fully (by watching the actuator rod) with no sticking or binding.
So the final step was to check the EGR system.
An EGR valve sticking (or blowing) open causes large amounts of exhaust gas to enter the inlet manifold and reduces the exhaust flow to the turbo, reducing the energy available for it to spin up and make boost. It also dampens combustion greatly, virtually extinguishing the flame in extreme cases.... and making black smoke. And finally it replaces the oxygen content of the inlet charge, and without sufficient air inlet the ECU won't put much fuel in, which reduces power hugely!
So I disconnected the vacuum hose from the EGR valve that sits on the inlet manifold and plugged the end of the hose with a 5mm bolt.
Problem solved! All the power is back and the smoke is gone.
I would get a new EGR valve, but as their sole purpose is to pass the legislated emissions test (which won't be tested again) then I won't bother. As far as the impact on MOT goes it will actually reduce the smoke output, so that's fine by me.... and as an added bonus fuel economy will increase as will performance at part throttle