I personally think that the Norexan is roughly equivalent in firepower to the D'Deridex. Judging by the readout that Geordi gives earlier (52 disruptor banks and 28 odd photon tubes?)... that's a helluva lot of firepower that the Scimitar has. Now, let's assume that the Scimitar has approximately 75-90%% of her weapons on the front, as would be tactically feasible (Alpha strike the target w/ all weapons while cloaked). Take the Norexan who is hit rather nastily by the Scimitar's weapons, Donatra's ship.
The vessel is punctured by a full salvo of torpedoes in the drive section... for strength indices, the Galaxy is obviously much weaker... warp core breach with a few hits from an outdated Bird of Prey. Here, a half dozen to a dozen torpedoes right in the engines and no breach. We also do not know how much damage the ships had previously taken. Of course, consider the "I want the Enterprise alive" approach.
Also, note the Viceroy's fear at the approach of Donatra's ship... he seems worried. This indicates to me that the weapons on that ship are to be feared by even the Scimitar. (Remember, shields at 70% after the entire battle). Plus, the damage done possible from the Norexan would have had to have been enough that Donatra would have believed that she could do sufficient damage to disable his cloaking device. Add to this that she says "Stand by disruptors indicates that she has more weapons ready to fire. She didn't get a chance to use them because of the Scimitar's stopping manoeuvre, which also hints to some powerful close-range weapons.
The possibility of this type of short range weapons can be further supported by the general manoeuvrability of the vessels. Obviously the enemy would not let the big, slow D'Deridex move into range and orientation to fire these supposed supercannons (which would seem to be located in the main pods beside the nacelles on the Norexan). Therefore, a new, more manoeuvrable ship is needed to bring the weapons to bear. Enter the Norexan, a smaller, faster version of the D'Deridex, more suited to space combat. We must after all remember that the bulk of the D'Deridex is taken up by troop transport space. This, when removed (think it was 16,000 odd troops), could drop the vessel size significantly.
To my mind, the Scimitar itself could not have been constructed w/out Romulan help, the Remans just don't have the freedom and materials they would need. This leads me to say that the Romulans were fully aware of the capabilities of the vessel and are, to put it simply, incompetent. Nearly every captain in Trek seems tactically incompetent, so this is no exception. However, I'll save my treatise on tactics for later. My hands are getting tired.
Of course, if the Norexan is indeed a manoeuvrable battlecruiser, it would not be feasible to send the D'Deridexs, even IF they were more powerful, they simply wouldn't be able to turn fast enough to keep the Scimitar in their lumbering gun sights. But then why not send a half dozen Norexans? Could be that they are just entering service and there are few around, but Picard (or anyone else)'s lack of surprise at any 'new' design for the Romulans suggests otherwise.
Could also be that warp speed has been sacrificed terribly, but this can be invalidated by travel times. Shinzon + Enterprise depart from Romulus at maximum warp (I might also add that the Scimitar is faster than Starfleet's fastest ship at max. warp., hinting that the Tal Shiar are not dead/useless, or that Romulan research is ahead of the UFP's). The Norexans arrived soon after the two above vessels at the Rift, clearly showing they have a speed comparable to the Enterprise, even if we say that they took no time at all to assemble a task force. Of course, this assembly time would be a factor, knowing the Scimitar will catch the E-E in a few hours, the Romulans dispatch all the nearest ships at top speed to intercept. These two, sent from Romulus with Donatra, arrived first.
Now, to anyone who read all that, give yourself a hand