According to this thread Done by AssaltWaffle, it takes 12 Gigajoules to vaporize 4 Zerglings. It was a well done calc and use to be a lowball, but there is now a new revision for vaporizing a human done by FanofRPGs. It only takes 300 megajoules to vaporize a human as opposed to 3 Gigajoules; which is actually the energy required to atomize a human.
Taking that new formula, 300 MJ * 4 = 1.2 Gigajoules of energy or 1,200,000,000/4,184,000 = 286.8 kilograms of TNT equivalent. Building level or 8-C. Still a lowball, but this will scale to the fodder tiers again; Marines, Hydralisks, Zealots, Zerglings AP, ect.
Note: Might be late, but apparently, Psionic Storm is atomization according to this scan, so the old calc might be applicable after all. So High Templar and those comparable to them may be upgraded back to High 8-C.