I’m sorry but you are wrong.
The stock plugs are BPR6ES. One colder is BPR7ES
If you have any cracks in the turbine (exhaust) side of the turbo you will see less boost and the harder you drive the worse it will get as with the extra load comes extra heat and the extra heat expands those cracks. When the cracks expand exhaust energy is able to bypass the wastegate flapper and thus bypass the turbine wheel. This sounds like more of the scenario especially because you said you didn’t find any boost leaks (assuming you checked properly).
While the knock sensor creating phantom knock is a possible cause, it doesn’t happen nearly as often as this does. Check your knock sensor to see if it has gooed all down the block and if it still looks whole check else where. if you have access to a logger you can check for knock while you drive.
I’m also willing to bet you have boost leaks somewhere, if you didn’t find them, we have open appointments right now, I guarantee we will find them.