When doing the cardassian mission where you have to destroy the True Way fleet along with a dozens tarfleet ships, I was one-shotted by a high-yield from a Cardassian cruiser right after respawning. Full shields, full hull strength. I see 4 torps coming at me, my shields shredded after the first one, and the other 3 killed me. This was with full Aegis gear at VA. I aso have to add, that when I launch high-yield III torpedos at a ship with full shields, it barely touches their shields, let alone their hull.
Having your hull killed by 3 High-Yield torps once your shields are down is not abnormal, but having your shields demolished by just one of them certainly is. My previous experience is that it has taken about three High-Yield Plasma torps to completely remove a shield quadrant (and when a D'deridex battle cruiser unloads its full complement of them at you, it often WILL kill you if you have no heal or resistance skills ready to use).