Working it through, the prop won’t drive the engine to over-speed and loose boost, as the CSU worked from RPM. Anything over the set 2,850 would cause the blades to coarsen until 2,850 was reached again, in the same way that anything under will make them finer. What’s providing the motive power and torque would be irrelevant to the CSU.
The blades are still supplying excess drag beyond design parameters as even at ‘zero lift’ they are draggy due to being locally above Mcrit near the tips. So the engine begins losing RPM, at an altitude lower than would be the case if the blades weren’t thick and causing this efficiency loss. Further, the blades are ‘flattening’ beyond aerodynamic usefulness, in response to the engine slowing.
Apart from the intake pressure difference caused by ‘flat’ (or even negative incidence) blades, I cannot account for boost variation.
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