I tested my Paranoid Delusion companion on my rogue. Here the damage he is doing according to ACT (with about 1000 hits):
No indomitable runestones: 5525 average damage.
Six indomitable runestones: 5543 average damage.
That's a damage increase of about 0.3%. Clearly the indomitable runestones, rank 15, do not work.
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If power was the problem, could have capped it at 180k , js