"For Discipline or Holy priests, spell penetration is not nearly as useful as it is for other casters"
Where are you getting this? Missed/resisted psychic screams lose matches...often times the healers are the ones being focused and that is the ONLY move you have to get people off of you. With the cd being as long as it is, you can't afford to have your psychic screams resisted. If anything, I'd say it's more important.
I want to challenge the following statement in the opening paragraph:
- In PvE, targets of a higher level than the caster have a base resistance (2% of mitigation per level difference)
If you check the Resistance page, it gives the formula for the effective mitigation as
Damage reduction = (R / (Lc * 5)) * 0.75
and the resistance score (due to level difference) as:
R = Rb + max((Lt - Lc) * 5, 0)
Therefore, for a mob that is higher level than the caster, and assuming a base resistance of 0 (Rb=0), the uncounterable damage reduction works out as
Damage reduction = ((Lt - Lc)/Lc) * 0.75
which, for Lt=73 and Lc=70, works out at approx. 3.2%, or approx. 1.1% per level. Hence I do not understand the 2% of mitigation per level difference statement.