This sounds similar to what I had on my Cos 4X4, my car wouldn't make more than 2 PSI of boost.

If you still have a standard exhaust, it could be the first precatalyst (the one on the downpipe coming from the turbo) that has collapsed/melted partially (could also be the other 2 cats further down the exhaust), just letting thru off-boost exhaust gasses.
When coming on boost the passage is to narrow and blocks the way for more gasses and the smoke returns to the plenum choking the engine.

(at the same time the membrane of the original Bosch dumpvalve was ruptured too, probably the fault of the melted cat, so i had to fit a new
dump valve too)

This cured my problem:

remove the downpipe, cut a little window/flap with the angle grinder in the cat, open it up and remove the material, then shut the window/flap and weld it up.

or you can fit an aftermarket exhaust without cats.

I hope this helps, with me it did

Kind regards