I really need some diagnostic help if poss - boost problems

[enkidu]

I posted a few months ago but still can't get to the bottom of it. Just need some suggestions if possible as i don't have a scooby what to try next.

Background.
Turbo died so i bought a recon and replaced
While changing i noticed my tandem pump was leaking so replaced at same time.

Now the car is lacking power below 2k rpm and randomly above that. When i floor car from say 1.5k it really holds back sometimes until 2.5k and the black smoke is crazy then it fly's. But if i start flooring it above 2k it feels fine, still low on power low down but not so much of the smoke screen, It's very intermittent.

I've decatted it encase it was a blocked cat but it's not that and I've tried adjusting my actuator shortening it but no change. I'm guessing it's a sensor or n75 but when testing on vagcom all seems normal. MAF reading up to max, boost reaching 2400 as it's remapped from about 2100rpm but obviously not below that.

Can the MAP sensor cause lack or low down boost? Or N75? Or MAF Even though they all seem to tes ok when on vagcom or could it be a boost leak?