Mk6 GTI Underboost Error

I've neatend up your Excel Spreadsheet, and I can see that your car made 19.23psi of boost. I suspect you have a K04 turbocharger to make that boost.
www.sensorsone.com/mbar-to-psi-conversion-table/
I would need to see your accelerator pedal input, as your actual boost is close enough to the actual boost where it counts. I would say that the turbo (in terms of boost) is not your issue. I'd need to see your fuel pressures (requested vs actual) and the LAMbDA (the one before the catalytic converter)'s reading. Bear in mind, you will never see the car reading below the 1010 mBar atmospheric pressure (although less will be requested).

I won't look at ignition timing until I can see your fuel(s), lambda, and pedal response. A single pull in 3rd gear should be sufficient once the engine is at operating temperature.

Graphing the boost in Excel shows that everything is alright in that department.
The BIG tell-tale sign of a problem is your N75 valve duty cycle being above 80%. Anything over 80% means a boost leak, but yours isn't too bad. You will see it on a dyno-graph, or feel it in a drag-race, but the leak might not be apparent on a spirited drive by yourself. A boost leak WILL mean a reduction in fuel volume delivered once the Lambda values are supposed to be about stock. To get those boost pressures, your car would be at least flashed, and maybe a bigger turbo.


I'd need to see your fueling and accelerator pedal, to help you make use of any of it.

I've attached the Excel Spreadsheet with sorting-filters on top to make it easier for others to read. You have a boost leak, but it isn't too bad. The leak seems to start around the ~4,000 RPM mark, hence its source will be difficult to spot (such as a slit in a rubber hose).