GT JTD sluggish high in revs
I've had my GT about a month now and when I test drove it I put the lack of power higher in the rev range down to how it had been driven by the previous owner, there was a collosal cloud of smoke when I booted it on the test drive (which has never repeated so I suspect it was never driven). However, although now less smokey (more on that later) the max boost/higher revs performance hasn't really improved.
1st and 2nd seem fine which I suspect is because you get through them before the turbo gets to max boost. However, if you give it a bootful from 3k rpm in 3rd or above (typical 40-60 overtake, accelerating hard onto a dual carriageway etc.) the performance doesn't build progressively to the point where it runs out of steam like my old Seat Toledo (130bhp) did, instead there is a whooshy sound as if there is a boost leak, but it is only there when it gets to max boost after a few seconds of foot to the floor.
I wandered if it was the apparently common bottom turbo hose problem, but Jamie at Auto Italia has had a look and he said it was fine. He took it for a test drive as well but on the lanes near him couldnt replicate the noise as it only happens in the higher gears.
Any thoughts on what it could be. I've checked the air filter isnt clogged and tried unplugging the MAF to no avail. The car's done 26,000 miles and there are no dash warning lights.