Nothing is known of the exodus fleet that left Vulcan and departed for Romulus. It could be that they utilized a form of FTL drive more inefficient or different from warp drive. The T'Plana-hath (from First Contact) after all has no discernible warp nacelles as we know it. It could be that it (like the Romulan exodus fleet) did not have warp drive. So, if Warp drives were more effective, they just stole the drive mechanisms and tried to replicate them as best as they could with their technology.
All there is to generate this particular line of thinking is a dropped line from Balance of Terror about sabotage and espionage, and the visual connection of two long nacelles, capped by glowing tips, held away from the primary hull by pylons.
Since the SFC1 manual talks about Romulans being little more than a galactic nuisance until they got warp drive (A/AM reactors, specifically), I thought it was rather supportive that the Romulans would be actively trying to steal stuff to make their vessels more powerful. Warp drive was better than the atomic-powered craft of yesteryear (discounting Enterprise's accounts, of course), so they stole it.
One thought is that they tried to use it as a primary drive mechanism (eschewing impulse engines entirely), but Scotty presumed they were using impulse reactors since the power output of the craft was nowhere near the levels of A/AM. Could be that the reason the Federation took so much interest in the Cloaking Device by the Enterprise Incident is because the cloak could cover A/AM powered vessels completely.