Some vague points that I don't think have come up are vaguely related:
Some fans (as well as old FASA manuals) also believed the Klingon Bird-of-Prey was a Romulan design being used by Klingons, presumably as part of the battle cruiser/cloaking device deal. Enterprise later gave Klingon's a near-identical bird-of-prey at the end of season 2, about a century earlier.
The similarities in the K and R ships could still be explained by a technological exchange, just bump it back long before Enterprise. Otherwise surely Romulan ships would/should resemble Vulcan ones?
It's always seemed weird to me that Klingons and Romulans both have green ships. With Romulans it makes sense: Romulan blood is green, and they want to appear scary. With Klingons...they want to scare Romulans?