If i have a sunfire cape equipped, will it deal more damage to minions if i have magic penetration on my items or runes? (will the targets have reduced armor from the magic penetration?)
Yes, just remember that you cannot use magic penetration to lower below 0, so if a minion has 0 or negative MR then adding magic penetration will not help.
However when i got my 2 sunfire capes on Amumu with Abyssal Scepter I have -60 magic resistance reduction, so the creeps do have a negative magic resistance and I do over 50 dps per cape.
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