I'd undo the hose clamp holding the air intake on the turbocharger and with a clean finger I'd give the turbo a bit of a wiggle - it shouldn't wiggle at all - and then try to turn it, it should be velvet-smooth, with no roughness in it at all. I havent had the pipe off the front yet due to work times and just lack of time to play, i have noticesd some nice oil staining on the pressure side of the front housing where the pipe goes to the intercooler, but having a day to play next week i will have a closer look at the turbine and see if it has any play...
At full boost your turbo is also sucking away like mad and if the engine bay is hot enough your air intake hose may be collapsing (I think some STs were doing this, and looks like the STX550 has this issue too). If this is it, replace as much of the air piping as you can with hard pipe. It's not a design issue, it's a cheapskate issue. yeah i been looking at the cheap piping and replacing it for the shear fact it looks crap and proper piping will allow better air flow, but never thought of the pipe vacumning it self and restricting but it is something worth a look
You might also have a stuck EGR valve, flooding the engine with EGR. Block that sucker off with a stainless steel plate (driver's side, takes about 15 mins to manufacture and fit).yeah something i dont wanna play with and no access to something to block it off out here so easily
There might also be an issue with the fuel filter/pump, but there's no way of telling that just yet. Has the fuel filter been changed recently?will be changed next service,,, if it was blocked it would lose power up top which it isnt doing, it still pulls like a cut snake
Speaking of filters, what condition is your air filter in? If it's caked in dust your engine won't breathe very well at all, and at those rpm it does make a difference.
