So I was doing some calculations. Important values are in bold. I know this kind of stuff has been done before, but I think this illustrates pretty well the perils of giving Unnatural WP to Psykers (IMO - the peril of giving easily obtainable Unnatural Characteristics to any player character - this is just an extreme example). Anyway, this pits a max rank Primaris Psyker against one (or more!) Lord(s) of Change.
Rank 16 Primaris Psyker, originally a Scholar-path Sanctioned Psyker. He got lucky and started with 50 WillPower (only needs 45 if he uses the Judged By Your Peers Transition Package). Boosts it to 80, gets Unnatural WP*3.
6 Psy from Sanctioned Psyker, +4 from Primaris. Push for a total of Psy Rating 13.
Power Well (2) = +2, Discipline Focus +2 and WP Bonus 24 = +28 to focus power roll.
Disipline Mastery gives Force Barrage Threshold 16.
Focus Roll: 13d10 + 28 - 16 = 25 to 142 overbleed value
/5 = 5 to 28 extra bolts, +24 from WP Bonus
29 to 52 bolts total.
80 WillPower and using the IHQ rule of +10 per 'past normal' unnatural trait gives 100% accuracy.
Each doing:
1d10+24 - 7 (Daemonic Toughness doesn't work against psychic powers)
Which is 18 to 27 damage per bolt
576 to 1431 wounds total, which can be split between targets
Divided by 180 (Number of wounds Marabas has)
Gives: 3.2 to 7.95 dead Lords of Change
This basically means that a max rank Primaris Psyker with a moderately lucky starting willpower, and some correct choice of talents and powers, when pushing, HE KILLS THREE, AT MINIMUM, TO NEARLY EIGHT, LORDS OF CHANGE, EVERY ROUND.
Now I appreciate that this doesn't take into account the LoC's scheming and influence, nor psychic phenomena, but it should be noted that this power is equally silly against other opponents (something like 21-62 Dire Avenger Exarchs every round) and fettering the power doesn't change the outcome much either (like 2 bolts fewer at minimum).