@Jolly Joker and
@Nick_S have good points. A countervailing restraint for Precision Striking a Mech’s Head might be a good compromise. Perhaps the difficulty in Precision Striking a Head as opposed to a limb or torso could be recognized by the condition that to PS at the Head, one only gets to fire a single weapon... kinda like the sniper taking steady careful aim with his rifle, as opposed to his rifle, his shotgun, his pistol, and his automatic weapon.
Still brainstorming, PS currently servers a number of functions. Early game it's occasionally useful just to boost the chance to hit without worrying about the ability to aim for a specific location. Limiting it to a single weapon would make it much less useful early game.
Broadly speaking PS helps with all of the elements of efficient offense: (1) increases damage by improving chance to hit, (2) increases ability to degrade the opponents capability by allowing targeted attacks, (3) makes it easier to concentrate fire on a critical opponent (by reducing their initiative).
Similarly vigilance (the defensive ability) helps in multiple ways (1) removes stability damage, (2) sets braced, (3) improvement to initiative makes it easier to get out of a bad situation the following turn.
Putting more limitations on PS would make it less of a general-purpose offensive ability. I think the approach should either be to split out the offensive abilities and end up with two offensive moral abilities (for example one which improved chance to hit and hurt the target's initiative and then other for precision targeting) or to keep PS as a broad ability and make it less useful for targeting specific locations.
Splitting it in two is less good. It requires changing the UI, and it's not a great idea to have two abilities one of which is mostly useful in the early game and the other is mostly useful in the later game. Better to have one ability which can be useful throughout the whole game.
Suppose Called Shot Mastery is really the problem, and that base PS is fine (which I think is true). What if high-tactics combined with PS functioned like a TAG system, rather than improving targeting? Tactics-6 could set a state so that all fire on the targeted mech would gain 10% damage for the next turn. Tactics-9 could increase that to 20%.
That would take away from the "target specific location" portion of PS but would enhance the "concentrate fire" aspect. It would also be in keeping with the name of the skill -- "tactics."
I like that much better than my first idea.