1.8TSI black smoke on full throttle

Would appreciate anyone's thoughts/suggestions as to the cause of some low performance/running issues I'm having please!

Car is a 1.8TSI with a K04 and appropriate hardware/software to match (S3 intercooler, 'sport' catalyst, large bore downpipe, remapped etc).

It's already been back to the tuners that built it, but they're stumped as to the cause, and set off on the course of just throwing new parts at it.

Symptoms

Down on power.

Black smoke on full-throttle.

Occasionally logs P0299 "Control Range not Reached", no other fault codes

Things checked/replaced

Boost leak check done by tuner

Mitivac test on turbo actuator done by tuner

MAF replaced

MAP sensor replaced

N75 replaced

Diverter valve replaced

PCV replaced

New spark plugs

Do these motors suffer from carbon build up issues like the earlier direct-injection VAG motors?

Cheers,

Andy