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. As for a too lean mixture two things will happen
1) The engine fuel flow is so restricted that there is not enough to support full combustion at max power and the engine will start to run rough. or
2) There is enough fuel to support full combustion but not at the over rich condition necessary to allow proper cylinder cooling at high power.....in which case you will get abnormally high CHT's assuming you did not catch the problem during the runup.
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.
However since from what I have observed over many years instructing most PPL's do not
1) Actually check the field baro indication prior to start up to see if the MP guage is reading a sensible number.
2) Do not know what they should be looking for in the runup particularly when it comes to testing the mixture control,carb heat,mags and cycling the prop
3) If they look at all at the engine guages at the start of the takeoff (which many do not) look but do not see.
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.
Finally if the aircraft is not accelerating normally it doesn't necessarily mean that the engine is not producing full power which points out the importance paying attention to what the aircraft is telling you and aborting (early) any takeoff which doen't feel right and for short fields having a physical go no go point allready established in your mind.