The thing with postnatal inhibition of myostatin is that you will not be able to achieve 40% increase in muscle mass no matter what you do.
You see, when there is a genetic loss of myostatin, the number of muscle fibre as well as muscle fibre diameter both increase (hypertrophy and hyperplasia), which gets you about 40%. However, after birth, of you manage to get rid of myostatin, you only increase the muscle fibre diameter(only hypertrophy) which is somewhere in the region of about 16-20% depending on which technique you use.
If you are looking at before birth alterations, might I suggest also looking into combined myostatin inhibition and follistatin upregulation ? Scientific literature has shown that muscle mass may increase by about 4 times.
Alternatively, if you are taking of a postnatal augmentation so to speak, I'd suggest messing about with the growth hormone, (though be warned that will screw up a lot of things, mostly making the individual more aggressive (as orcs are meant to be).
Also note that myostatin affects the fast muscle fibre more than the slow types. You use fast types for quick burst activities and the slow types for well.. endurance. So myostatin inhibition may actually be detrimental for endurance training (also causes a fibre type shift, converting some slow fibres to fast fibres).
Do note that when you increase muscle mass like this, you are not necessarily increasing quality. You will be increasing the total force output sure, but the muscle on the whole becomes weaker (specific force decreases) and there are cases of brittle tendons and other things.
You make a few good points but
I must note here that Myostatin inhibiton has been shown to have no increase in the size of the heart (as its a different fibre type).
Muscle stem cells are not affected in any way by myostatin inhibition (acts by a completely different pathway)