What effect would (say) a burnt valve, fouled plug etc., have on that manifold pressure? My gut feel says nothing.. the MP gauge is reading just that - MP. If the engine is spinning at the full RPM, the MP wouldn't be affected by the amount of power the engine is generating, it's still pumping the same volume of air, right?
Less sure for a turbo'd engine, but the logic should be the same.
P.S. Ditto the fuel flow - both measure the inputs, and don't really give a clue whether there's power being produced, so long as there's enough oomph to get the prop into the governing range.