Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.
Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.
So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.
That is rather the point - for a 'normal' smooth running engine there is a great correlation, absolutely. Nor am I setting out to find obtuse .1% probabilities to pick holes, or be pedantic. Maybe it's a burnt valve, or just some fouled plugs, but lets consider the dead mag....
In the case of a dead mag I can say from personal experience that the engine will not develop full MP because I aborted a takeoff in a twin last year after noticing in the "good engine check" the MP needles were split. It turned out the mag coil had died.
I have no knowledge or experience of turbocharged aero engines, but I guess your twin in this example must be - that is the only way I can conceive any feedback from combustion to the manifold pressure (altered combustion could affect the exhaust, which might affect the turbo, which might in turn feedback to inlet manifold pressure).
However, I cannot understand how it would make a difference for a normally aspirated engine - if it's turning full RPM, it sucks a set amount of air. The MP gauge has no clue whether the air is being used to oxidise fuel or not. If you could spin the prop at redline rpm on the starter with both mags off, it would show the same MP as it would running normally, and probably the same fuel flow - but CHT / EGT would be zero.
However, I guess in thinking it through I've answered my own question, the only thing that will tell me something is wrong is the seat of my pants.. EGT might give some clues - if it reacts fast enough (and yes, I know to look for a rise in EGT on one mag..)