The Romulans are supposedly many millennia past warp speed. They use singularity drives which are arguably a higher form of technology than matter-antimatter combustion. The Federation steals a Romulan cloaking device in TOS and is astounded by the capacity of their "plasma torpedo" weapon in "Balance of Terror." In Enterprise the Romulans use long range drones with holographic technology to change the kind of ship they're flying centuries before the Federation seems to have holographic technology.
By the 24th century it seems that the Federation and Romulans are pretty much evenly matched but I would argue that the singularity engine is actually a more advanced technology than matter-antimatter reaction.
As I said, their ships run on singularities - they are evidently quite knowledgeable about them. I don't know why Spock would know something about this "red matter" that the Romulans don't given their apparent vast experience with creating artificial black holes.
Arguably, we just don't know about the engines. But Romulan ships seem to be no faster than their Federation counterparts. Plus, we don't know that they are many millennia past light speed. Their flight from Vulcan could've been done in sublight craft (as conjectured in the Vulcan's Soul novel series). We also don't know how long they've used singularities. But LaForge and company didn't seem too impressed by it, if I remember correctly.
And whose to say that Romulans don't know about Red Matter? Knowing about something and being able to make a working item are two distinct things.
I don't see long range drones as being a game changer. We see robot ships in TOS.
The plasma weapon was obviously a failure as it came and went pretty quick. Making its final appearance in "The Deadly Years".
All in all, I'd say the Feds and Romulans were on pretty equal footing where technology was concerned.