Comprehensive replies Ged, thanks. I concur, you can't be in the copilot's seat for every flight, the pilot must bear some responsibility for the events of the day. As you state, he was able to verbalise and record on training documents the MP he should have expected to see that day -but still persisted with an attempted departure despite the obvious shortcomings in performance. Those that suggest "mixed-fleet" flying as a defense/excuse... puhleeze! The pilot would have been approved to fly that (turbocharged) aircraft as well as any normally aspirated airframes and would have been well aware of the differences in performance of the turbocharged airframe. On turbo'd airframes I have flown, you will comfortably hold the aircraft on toe/park brakes up to about 29"MP... but once the turbo kicks in, she's going somewhere, regardless of what restraint you have trying to hold her. It just ain't possible to mistake the difference between a functioning or non-functioning turbo.
The suggestion posted earlier that "the aircraft will still fly without the turbo" (or words to that effect) I believe may refer to an aircraft already airborne (in the cruise?) at the point of turbo/waste-gate failure -assuming of course that the failure is not catastrophic, throwing red-hot bits of high-velocity metal all over the place. Certainly not a "take it flying without the turbocharger" situation.
Those suggesting this was a "soft" failure -wow, I just don't get your reasoning. There's not a great deal more attention-getting than an airframe not performing to expectation under full-power conditions IMO. I have aborted a departure in a BN2 with a 2-3" discrepancy in MP between the engines at full throttle once. It could have been attributed to instrument discrepancy in an old airframe, but she didn't sound/feel right. BB probably remembers that incident -it turned out to be a carb-air box that had worked its way loose. Another time a former CP aborted on a very short runway with a full load under similar circumstances, also in a BN2. That time it was a mag-switch failure. If something doesn't feel right, sort it out!!! Your arse is a pretty good indicator of trouble -as well as being what you are putting at-risk (as well as any others attached to your airframe) if you don't.
Keep up the good work Ged, fight the good fight. I believe there is beaucoup industry support for you, personally and professionally, not to mention sympathy for the circumstances thrust upon you.