mephisto is right. The acid inject isnt affected by any +% or +dmg mod. Its a plain and simple "curse"-based spell that lowers your liferegeneration. And liferegeneration thing is put on a number that cant be changed. But its possible to lower the duration with fade for example. Cuz its curse based

If your merc is wearing the acid injector, the damage of the spell itself isnt soooo much... What really kicks in while this "curse" is active is the fact that it lowers the monsters physical immunity to absolute zero. You will do maximum damage for around 5 seconds. And most of the time its refreshed while its active cuz the bow-merc strafes alot

Since this is a curse you may not want to have lifetap ctc on some items. So switch draculs to whatever statstickgloves and change exile for zaka or something with high damage.

infos on acid injector:

http://annihilus.net/wiki_index.php?title=Acid-Injector
(the comments here are interesting)

http://annihilus.net/wiki_index.php?title=Acid-Injector
(stats of the bow on actual content)

http://annihilus.net/showthread.php?t=8375
those are the 2.3 patchnotes with the comment from alex:
• Acid Injector and it's skill Inject Acid have both been buffed. Inject Acid now lasts roughly 5 seconds (up from 2) and now also removes all of the target's Physical Resistance / DR % and lowers their armor by 100% for the duration of the curse. The Inject Acid change is retroactive and will affect existing Acid Injectors, but the bow itself has also had its stats buffed and will need to be found again.

----------------------------------

And to get back to topic:

in the character guides section in the forum (not the wiki) there are actually a few different playstyles for smiters.
One managed to make a poison based smiter with the lichking dagger

and @Dante im pretty sure, Frostmourne is not the best weapon to do especially dreamlands... Sindragosa procc is nice but against bosses its the best to have plain physical damage and mass CB.
For sure on inferno and plaguelands frostmourne rocks and if the smiter has no problems at all you can just use frostmourne all day long.

Good alternatives are the lichking dagger (but has no cb nor stops it monster regen)

or Heart of the shadow. My favorite due to amp, good +damage, good speed and good cb on it...