Pierce>fire conversion can be found on many items; Vilgazor’s Heart provides %60 by itself which make other items obsolete. Moltenclaws, Cinderplate Belt, Light’s Oath ring, Ignaffar Gloves and Korvan Armor provides this conversion. Interesting thing is only Cinderplate and Ignaffar gloves are AoM, nevertheless it’s enough to get %100 conversion without FG.
I know this particular amulet changed a few times already since 188.8.131.52 but phys>fire conversion is rare because no exclusive skill provides it; and this amulet became mandatory for almost every melee fire nightblade variants due to acid>fire conversion.
So my suggestion is, keep the mods on pneumatic burst but change the %20/30 pierce>fire conversion into %8/12 physical>fire.
PS: Please warn me if I’m missing something and don’t hesitate to share your opinions on the matter.