QUOTE(decondelite)
You may cast everything without triggering a single channeling as much as triggering it successfully...you're losing the mana that IA would have saved.
If you do have IA5... there is nothing to discuss: don't ever do that. You'd lose pretty much the equivalent of the cost of Heartseeker itself by hardcasting, as autocast buffs have a 50% reduction on the upkeep cost.
I tried to crunch the numbers on myself just now but it's hard to decide the best way to compare. Without Spirit Stance:
Protection = 100 MP (210 turns)
Spark = 124 MP (125 turns)
Haste = 138 MP (125 turns)
Shadow = 133 MP (98 turns)
Spirit Shield = 119 MP (104 turns)
Heartseeker = 469 MP (352 turns)
Regen = 281 MP (102 turns)
Cure = 80 MP
Absorb = 119 MP
Total Mana = 1634 MP, +27 MP regen per tick
The
Wiki claims the estimated chance for Protection + Spark + Haste to proc Channeling is 362 / (1634 * 1.2) = 18.5% though that seems low to me.
Using IA5 the cost saving is equivalently 362 * 50% = 181 MP
With hardcasting the savings are 18.5% * (469 * 150%) = 130 MP
It's a close comparison and IA4 is about the same either way.
QUOTE(Paruri)

Can element mitigation be minus?
It is important to note that mitigations are applied in a multiplicative fashion. However, they cannot be reduced stats below 0, or reduced further if the mitigation is already negative. WikiWait, I'm not convinced the wiki is right. I agree with the wiki for physical and magical mitigation, but I always thought elemental mitigation is additive (but multiple elemental mitigation effects are multiplicative with each other). And I believe elemental mitigations can be induced negative. Otherwise, why do melee styles get a 5% boost to all damage on Friday (void day)?
Maybe later I'll test it, but it's not such an easy test. I need to use a similar method to my
void day discovery.