First and foremost will be Invisibility no doubt, it is due to not only will it make a good unit ultimately terrifying (imagine you have an Invisibile Imperial Knight charged into enemy valuable unit

or your Grav cannon Kataphron Destroyer that canonly be hit on a 6s), but also it comes from a discipline that contains other decent powers (i.e. Psychic Sherik, Terrify, and Shrouding).
I think Preciense may not be that vital Because there is one of your Cantical allows reroll to hit, with War Convocation, you are almost garanteed to have 8+ units right?
Warp metal armor is good, but fishing for it risk rolling on a discipline which is very situational that may not help you in specific game, sure that haywire beam and the power that stripe of D3 hull points is good when you fighting an army of Tanks, but if you rolled them when your opponent is not bringing vehicles, those powers are dead weight.
There are two forms of Ignore cover Psychic power for
SM, in view that your major army is Admech, I agree on the choice of fishing phase form. Due to the fact that your army is comparatively slow, the discipline that phase form comes from also contains the infamous "shifting worldscape" you have a chance to get the power which boost your movement as well.
Some of the target specified recommendation are as follow:
Facing Eldar Seercouncil or Daemons? Roll on Librarius discipline, for Veil of Time, Psychic fortitude, Psychic scourge and Null Zone.
Facing high toughness unit with good armor saves (i.e. WK), try fishing misfortune combined with massed Radian Carbin
Facing hordes you may want powers from Pryomancy, the Fulmination witchfire can work, but not very reliable.