I would guess they're just as powerful against any ship - the way I figure they work is that their explosive warheads are not much more powerful than an ordinary photon or quantum torpedo, but they phase right through a ship's shields and hull and detonate inside - a multi-megaton blast from within will take out damn near anything.
I don't agree on the explosive yield bit because we have evidence that suggest 1 transphasic torpedoe would have to be several times as powerful compared to a standard photon torpedo.
1 Photon torpedo that detonated inside a Borg 'scout' ship (actually, designated as a 'probe') was enough to destroy it ... primarily because it detonated near the power matrix and the general goal of Voyager's crew was to disable the vessel, not destroy it.
On a second occasion, when they transported a torpedo and detonated it from inside a Borg sphere, it created a huge explosion and heavily damaged the vessel, but did NOT destroy it.
Borg cubes are far larger compared to spheres, and more durable.
You'd likely need to beam at least 4 to 6 photon torpedoes inside a cube to various locations in order to completely destroy it since Borg ships can operate even with over 70% of the vessel being disabled.
Since 1 transphasic torpedo was enough to eliminate a cube, there is a possibility it's explosive yield is equivalent to about 4, 6, or 10 photon torpedoes, and the effective explosive radius larger by a large amount.
Transporting a photon torpedo inside a 'regular race ship' would likely cause extreme damage and destroy it because they aren't Borg ships, unless the explosion is contained somehow with internal forcefields (although how much of the blast they'd be able to absorb is unknown) ... but again we can also speculate that the interior is made up of the same materials like the ship's external hull which could also prevent immediate destruction.