Hi! Thanks for your feedback.
Mine situation is slightly more peculiar.
Was a 156THP that was upgraded to a 175THP turbo. This upgrade was matched to a larger intercooler, hi-flow cat, Bastuck exhaust. Also remapped to approx. 200bhp with this setup.
Being hungry for more power, a hybrid turbo good for 250bhp was sourced.
Prior to fitting this 250bhp turbo, several items were changed/done - leaking engine valve cover+intake pipe, new turbo vacuum pump, spark plugs and also intake valves sandblasted. The 175THP turbo ran very well with the afore-mentioned. The remap was running at 20-21psi max with brisk acceleration and feather light throttle response.
The new 250bhp turbo was fitted, but it refused to go to 20psi! It gets to 10-11 psi and gets P0299 (underboost) and goes into limp mode.
No air leaks, diverter not faulty, turbo is not faulty. Yet it underboosts. Quite frustrated.