Here.
I've noticed three interesting quotations, one from "Thousand Sons" (which would indicate a firepower per lance around 1 MT, but that's an "at least") and the other from the yet to be published "Nemesis", the later quotes quite supporting the excessive number touted by Connor, and actually completely ridiculing the 560 GT torp. I wouldn't exclude the possibility of James Swallow, in need of some confirmation that looked solid enough for someone not knowing anything in that department, having run a quick Googleing and picked the most "influential" stuff he'd stumble upon at a first try that looked technical enough.
The quotes from "Nemesis" are of the kind indicating a degree of firepower for basic weapons that completely outclass the supposedly end-of-the-world level of power delivered by an Exterminatus operation, the kind which burrowed creatures, tough Nids and a Titan managed to survive to. An operation that always takes
hours to be completed with specialized weapons, and yet which is bested by the mundane weaponry config of some battlebarge or a Chaos ship of similar firepower. I mean, when your basic weapons can boil off oceans and crack crusts...