thank you for taking the time to respond, read, and think about this.
Although in the real world such principles seem obsolete (we just buy a turbo and use a tial and call it a day) its good practice to re-visit these sort of problems.
I make this quick in paint, to show what I am saying instead of just a wall of text
Note the gate shares the same volume as the turbine. So when the downpipe volume grows larger, and the turbine isn't using any of the extra space (same wheel speed with even less pressure)
all that extra volume can be used by the wastegate.
... And just FWIW. None of this really matters to a MAF car. Boost isn't measured by a maf sensor, only mass flow is measured by the MAF. SO only cars with MAP sensors care about the actual boost number.
In other words, if the engine overboosts from 12 to 18psi, the MAF has no clue there is any over boost condition. The maf does not care about that. The only thing that matters is whether or not the MAF can support the mass flow, and whether the injectors and ECU can fuel the engine properly at that mass flow. To put it another way, 12psi can make the same power as 18psi (same mass flow) due to differences in intake/exhaust restrictions, and to a maf sensor in that situation,
12psi = 18psi, it can't tell any difference.
If there is some issue with fueling that mass flow (lets say the boost and mass flow is climbing too high for comfort of the fuel injectors),
the easiest 'fix' (instead of upgrading the injectors) is to
use a smaller air filter, or add a restriction plate to the inlet.
This would be similar as a restriction in the exhaust (smaller diameter downpipe) without the negative influence on exhaust gas temp and pressure that a small downpipe volume creates.