Actually they still had the old Warbird model from Balance of Terror when it came to making The Enterprise Incident, but it was so hideously underdetailed that the producers said make a new model and you can film the episode, but we're not gonna pay for a new model, so being the ingeneous half-arses they were the VFX guys with their budget of $0 just took the purty new D7 model they'd just finished and used that.
Personally I much prefer the SFMuseum explanation of Romulan warp drive - the old ships were fusion powered and they only got M/AM reactors when they bought the Klingon ships - I don't care what anyone says it is physically impossible that the Romulans had a war with the UE and her allies without FTL travel, it just couldn't happen if the UE had warp drive as we are told.
The SFB line of the Romulans continuing to buy Klink ships was because they simply didn't have the tech or money to build anything near the level of the Klink ships untill they came up with the Sparrowhawk.
Completely OT now but hey: the Sparrowhawk is one of the but-fugliest things I've ever seen - Steve Cole I think designed the SFB ships? Anyway I'd be ashamed if that was the best I could come up with, though the ISC and Andromedans looked cool.