In WWII, torpedo boats, destroyers and cruisers all had roughly similar torpedo firepower, which made all the difference in certain styles of combat. One could hardly have claimed that a torpedo boat was the match of a cruiser in general terms, but one could still factually claim that the torpedo boat was equally heavily armed in special terms.
Not really. What one could actually claim was that a torpedo boat had enough firepower to SINK a cruiser if it got into torpedo range, which is essentially true. It's worth remembering that submarines were originally just submersible torpedo boats themselves, and for the entire first world war and part of the second they made most of their attacks while running on the surface, even using their main deck gun as a primary armament. In those cases a submarine (which, let's face it, is really what we're talking about when we refer to the Defiant) was by no means as heavily armed as a cruiser or even a destroyer, but with a smart enough commander it could sink dozens of ships without ever taking a scratch. It was only in World War-II and after that submarines really emerged as different class of combatant, and later with the advent of sleek underwater hulls that submarines stopped surfacing altogether and made all of their attacks from under water.
Since Defiant, like WW-I subs, cannot fire while cloaked, the fact that it has the torpedo armament of a heavy cruiser just makes it an ideal ambush predator. Otherwise, what you're really looking at is a Type-VII U-boat that happens to have a freaking enormous deck gun on the front of it. You could ask whether or not it can take on a heavy cruiser in a stand up fight, but stand up fights aren't what U-boats were designed for; you might as well ask how well the cruiser would perform in a dogfight. The full range of its capabilities can't be expressed unless you're using the ship to its specialty: since Defiant is an ambush predator, then it stands to reason that if it manages to sneak up on and ambush a Sovereign class starship, it will blow it out of the sky.