it could be the fact youve gone from a 120hp car to a 350hp monster and your feeling the turbo lag, but if its throing a cel and smoke out the back its probably not

sounds like your car is bogging. i doubt its a boost leak because it would happened all the time under wot. also check under the intake pipe and look at the ecu wires to make sure none of them are frayed. (thats what happened to mine)

checking what code the cel is throwing might help you a little bust most likely it will be emission system related.

wastegate problem would be cause by the flapper being partially open or shifted the wrong way. you wouldnt be throwing smoke out of the back for that tho or giving a code.

sorry i didnt anwser the question really but i can only guess what the problem is
(shoulda had him fix the problem before you bought it imo)

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