did you look at the up pipe ?? i had what sounds like very similar issue; sluggish, no boost, no power.... i was set to change the turbo, up pipe & downpipe. step 1 was remove up pipe and the honeycomb material inside was loose and could restrict the pipe. to me that would have caused the lack of power. i did however change the other parts. and now its a much livelier car.
so maybe it might be worth a look.