when it goes down on power does it reset if you turn the engine off then on again? sometimes something called the vnt mechanism sticks making it overboost and go into 'limp home' mode.
if your looking fir a turbo upgrade, i went for a pd130 garrett gt1794va turbo. but i had to fit a pd130 inlet pipe on and pd130 boost pipes and intercooler. makes it a fair bit faster