Another mechanic told me that my silencer is restricting the exhaust too much and could also be doing something there (3.5 inch turbo back exhaust, silencer takes it to about 2-2.5 inch at muffler).
Which is it, 2 inches would be a massive restriction, have you considered fitting an Exhaust temp gauge. If the tuner has to severly retard the timing to prevent detonation, then the EGT's can go through the roof. This would make sense, as the gtt's are renown for having to run low timing to compensate for their different combustion chamber design, and higher comp ratio
How close is the cat to the turbo, could try moving it back a bit.